Back Pressure? questions

[MRBP]

ok guys, i have an 07 6.0 with an age stage2 intake, MRBP 4' downpipe and 5" exhaust straight piped, and im running erics x-street

what does back pressure do? does it effect your preformance? if so how?
also my girlfriend just bought me a stack that i would like to put on my truck as a exhaust tip but it has a 4" inlet and i need a 5" would reducing down from a 5" to a 4" then to the tip be a back thing or will it be ok? thanks guys