I do use the 3" Flexwatt on my racks. I find that it does a fine job for me. Also remember that unlike alot of our members, I have a building seperate from house that I house my snakes in. The ambient temperature of that room is 85 degrees. It is pretty sweltering in there. I don't think anyone keeps that house that hot. So, the heattape is not trying to heat a tub from 72-75 degrees(ambient) to 95-98. That might be tough for 3" tape. My tape only has to heat up from 85 degrees.
p.s. I said 95-98 degrees, and before I get questioned about that, that is the temp on the tape, not in the tub. That temp is required to get the desired 93 degrees in my tubs.
In conclusion, I would say that each setup requires a look at the ambient temps, and what you are trying to achieve with you heat tape to best determine what tape to use. Also, if you think that the 11" is too big, there is also a 4" variety available.
Hope that helps,