It is worth mentioning that the use of a x 2 converter on your 550D will lose 2 stops when using the converter with a 70-300mm, the view finder will become very dark … it will magnify any faults in the lens, as well as increasing chromatic aberration, images will not be as sharp and you will probably lose some, or all electrical communication between the lens and camera.
Using a converter as well as extending the zoom range, the minimum focus distance is retained which would be a good thing on a fast lens …
Extension tubes are relatively cheap (unless you get the chipped versions, that retain the communication between lens, tube and camera) and would perhaps have more use than the converter when used on your zoom. Unlike the converter there is less drop off in lens performance because there is no glass in the tube, you will still lose light/f stops but because a zoom moves its rear element forward and backwards the loss will be perhaps more variable and more so if you have more than one tube attached. With the cheap tubes because there is no communication between the lens and camera you would have to alter the f number manually … I believe on Canon cameras this is done with the lens initially attached to the camera, setting the required f number and then holding in the DOF preview button while removing the lens … you would then put the tube on and then the lens.
As a side note chipped tubes start from about £60 for s set of three to something like £125 for a single Canon tube … I’m guessing that Canon air must cost a hell of a lot more than the air that fills some of those third party tubes.
Another option if you have a prime lens (Probably something in the region of 50mm to 135mm) is to get a macro lens reversing adapter ring, they cost roughly the same price as tubes and a lot less than a converter and will allow you to put the lens on to the camera body back to front and achieve close to macro capabilities.
It may be counterintuitive, but the shorter the lens' focal length, the greater the magnification. This approach also works with zoom lenses so you can adjust the image magnification without changing lenses. Also, the working distance between the front of the lens and the subject will never be less than about 40 mm regardless of the lens design, if you stick with EOS mounts, but these are probably about the least amenable to this treatment because you will lose all electrical communication between the lens and the camera body unless you get a special connector for the purpose. Because EOS lenses don't have diaphragm rings, you have no easy way to control the f-stop. However, because you are attaching the lens via its filter threads and not the lens mount, it doesn't much matter what kind of mount the lens has. You can pick up suitable old lenses for cheap, including Canon FD mount lenses. An actual diaphragm ring would be very helpful with a lens bought with this purpose in mind, but these are almost universally included on old lenses.
By the way, extension tubes and lens reversal "lose" light in the same way that behind-the-lens teleconverters do. The light isn't actually "lost" but the same amount of light must be spread over the bigger image when the image is magnified. The light loss is easy to compute with the teleconverter; not so easy but still possible with the extenders and lens reversals. For a 1:1 magnification, you can expect to "lose" two stops of light, i.e., the image would be one quarter as bright as a shot taken the normal way. For higher magnifications, you can expect to "lose" even more light. I can quote the formulas if you like.
Some other things to keep in mind:
Although extension tubes don't have glass in them, most general-purpose lenses are not optimized for macro work and the path through the lens is not quite the same as it is when the subject is farther away. You can therefore expect some additional aberrations to show up when using general-purpose lenses for extreme closeup work, including increased geometrical (barrel/pincushion) distortion and curvature of field problems. However, the image will most likely still be much better than using auxiliary closeup lenses.
A possible exception to this rule can occur if you about another camera lens backwards as your closeup lens in front of your primary lens. Mounting a second camera lens in front of your primary lens is likely to cut off the corners of your image, but may otherwise give a better image than simply reversing a single lens. The 'loss' of light problem also doesn't apply for close-up lenses, plain or fancy. (I won't explain why not here, but only the diaphragm in the main lens affects the exposure. The one in the close-up lens affects only the field of view. Closing that one down doesn't affect the amount of light getting to the center of the image but does affect how much the corners of the image are cut off. You therefore want to operate with the diaphragm of your close-up lens wide open, assuming that the diaphragm is manually adjustable. Unless this is also a macro lens, the focus on this lens won't matter much but should generally be set at infinity. (Then again, if you have a real macro lens, why are you messing about with this closeup lens stuff?)
I also have some doubts about the minimum focus distance being maintained when you use a teleconverter. I'd like to check that one out first.
