Using different string materials can impact speed but also affects durability as well. There is always a trade off. As for your KE levels....I would head over to www.bowjackson.com and use his KE calculator to determine your levels. If you are shooting cut on impact heads or even some of the chisel tips style broadheads then I would shoot for at least 55 ft. lbs of KE if you are going after some of the larger game animals.

Hope this helps.