How much power is lost through a 518

The benefits of OD outweigh the loss of extra weight when you factor in momentum, fuel economy and the engine spinning a lot slower.
Plenty of people here have installed the OD trans with great results.
Loss of HP? Not sure as it is not really going anywhere unless you have slippage.

Try it out and i believe dependent on the floor a few modifications need to be had to fit the darn size of the trans in place.
Good luck!