I use a 2xtc quite a bit on the back of my tamron 90mm macro with good results - this was taken with this set up last year
but its about the only lens i do use it on - you would certainly be disapointed by the results it will give on a 90-300.
if you do buy a 2xtc to go on your macro you dont need to spend oodles of cash - mine is the jessops own which is 39 notes
i agree with jason that an ext tube is also well worth a look as well or instead - jessops have these also for £79 for a set of three