after market heads on 5.7 vortec

I have been having a hard time replacing the heads on my 1999 tahoe. This all started over a bent valve. I was talked into after market heads. The company that makes the heads is RHS. I purchased a set of heads that have larger intake and exaust valves, and smaller combustion chamber. (THAN OEM) Now, I must say it isn't throughing any codes, but it runs like crap. Like it has no power, and burns through the gas. If anybody has done a swap like this let me know what I can do to get my tahoe running better