I am a huge believer in going with a Code 6 or Code 7 main for fractional rigs in PHRF....pays huge dividends up wind and down....only downside is that to properly fit it you typically need to mount a backstay flicker...although with a swept rig that is less of an issue then with an inline rig.
Your sailmaker (Dave) is bang on. I say go for it. Go with a Code 6 main and stay with a Code 5 headsail and your crew will also thank you. I seem to recall one of the Frac, rigged J/29's (Rhumbline ????) did the same idea and seemed to recall they were fairly happy with the results.
