I can't think of what your power problems might be, but I may be able to help with the tensioners.
This is what my team ran in 2006. Personally, I think this design was our best; it was definitely much better than our 2007 ones (our 2008 tensioners aren't fully made yet, so I don't have an opinion there). The u-bolts on the 2006 machine were easy to adjust and simple to make, in addition to being relatively cheap. For the actual idler sprocket itself, in 2006/7 we used ball-bearing steel idlers from McMaster-Carr, which worked nicely; however, they were kinda heavy for their function. So, this year we have aluminum ball-bearing idler sprockets from AndyMark, which only cost about $2 more and are a little larger, but weigh significantly less (sorry, I don't have hard numbers, we weighed neither the old nor the new idlers).