I have the same concern you do about the fine line between muscle building and being tormented. These fish are delicate by nature, and when stressed, they definitely tend to come down with diseases more readily. But, muscles and general conditioning is good for *all* living things, fish included, so the others do have a very valid point there. :) Stronger fish are generally healthier fish. :)
My guys all live in the equivalent of 10g tanks (they're larger tanks, divided, but everyone has their own 10g space), and it does take them a little bit to become accustomed to more space and more swimming room. My VT's and CT's definitely fare the best in that regard, they plop right in from their little cups and proceed to explore like Jacques Couseau. ;) Up and down and all around for hours. :)
I think it also depends on the type of betta. I cannot imagine making my poor EE try to survive living in a "currented" tank. He would die, I can almost guarantee it. He has enough trouble in his 10g's and a sponge filter, and he spends most of his time laying in his plants and lounging near the surface. The stress of trying to combat a current would almost certainly kill him. Of course, he is about as far from a "wild" betta as you can get...
Now, a VT or a plaket, they would probably do pretty well in a tank with a current, because they're not being weighted down by the fishy equivalent of a full crinolined velvet ball gown. Imagine trying to swim in 75lbs of water-logged clothes *all* the time. You would eventually build muscle, if you did not first die of exhaustion. I'm not saying that you shouldn't build muscle, but you would do it by degrees. You'd start out with a smaller weight, and then slowly increase it as your ability to handle the load would increase.
Your fish cannot take off his fins (except my double tail who just trims the edges off himself with his little mouth ;)...), but you can change the filter flow. The filter will still work baffled with sponging with no problem, the additional sponge will actually make the filter more effective from a BB perspective because of the increased living space for them. :)
The mechanical (particle) filtration will not be as strong, but I you're cleaning the tank and vacuuming the way you should, this will not be a problem.
I'm not familiar with your filter, but if you can fill your filter box with sponges (just take out the carbon, you don't need it, and some people believe it can be harmful because it absorbs minerals and other nutrients from the water), and the flow will most likely be greatly diminished. If it's *still* too strong, you can take additional sponging and rubberband it across the outflow on the filter, so that the outflow is completely covered in sponge, with the tail end of the sponge hanging down into the water. This usually will decrease the filter flow to the point that there will be no current at all.
So, by baffling, you can keep your filter, he can get stronger and healthier, and then by degrees, you can increase his current and allow him to build up to the desired level of current. :)
Best of all worlds, lol! :)
I hope this helps! :)