First off the TC6 IS a better car, why else would they have a newer version if it wasn't better...
That being said the TC5 can hang with anything current with the right setup and good driving... The battery DOES need to be moved outboard to get better weight balance but done right there does not need to be a lot of weight added at all to balance the car. The advances on the TC6 make it easier to drive fast but unless you have the skills of a national/world contender you can be just as fast with consistent driving and a good setup... When I got my TC6 i translated my setup from my TC5 straight onto it and it was instantly just as good but easier to drive...