If Ichnage my poundage from 55 to 58, will it change anything else on my set-up? Also what will I roughly gain in speed. Arrow weight w/broadhead is approx. 354 grains...

Well, a three pound gain shouldn't change much of anything except how much effort you have to use to draw the bow. Your fps gain would realistically be about 5-6.

fps 4-8fps roughly gained.... as said above, drawing effort increases

you will probably shoot about 2inches higher at 20 yards also and a slightly flatter line out to 30