Why don't you pick up a dedicated Macro lens? They are much better for such things...
Check into the Sigma 105mm, Tamron 90mm, Canon 100mm, Sigma 150mm....
They will likely have a faster f stop then any telephoto, and will give you much better detail. Also when you are zoomed out without IS it is very very difficult to get a sharp photo.
I have been in a similar place for a while with a super zoom 28-300mm Macro lens, but the shots are not that crisp. I just picked up a 50mm sigma macro, and the details difference is amazing.
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Canon T1I, Tokina 12-24mm, Sigma 18-50mm, Canon 1.8 50mm, Tamron 28-300mm VC, Sigma 50mm Macro, Speedlite 420ex
my flickr | MacroReverse.com - blog I run about reverse lens macro photography
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