I've got Rifle 5.5s on my irons and have no complaints whatsoever. Have you tried irons with graphite shafts before? They play very differently from steel and personally I don't like the feel of them (or lack of). The only advice I can give is to try the same set of irons with steel and graphite and see what you think.

Given you swing speed I'm wondering whether a 6.0 is a bit high but I'm no expert on this. I'm sure Graham will help out if he reads this.