I just installed 6.5 Auditor 3 ways (made by Focal) pushed by a RF PB300x4. Gain is only at 2 and it f'n rocks. Tell your buddy to get Scosche SAVW6 6.5" speaker adapter rings. They are made for a volkswagon, but work and are only $10 instead of $50 buying the same thing from companies selling them bike specific. I've got just over $300 in my setup, and its the best money I have ever spent on bike audio. I think the biggest he can add in his tourpak is 5.25, but not sure,. never researched into them.
Yeah biggest in your pack without cutting is 5.25. But I believe Kickers are the only ones that'll fit. They are shallow. Other 5.25's are deeper and then you have to cut the inner liner to make room for the magnets. I did Polk 5.25's in the front fairing and 6.25's in the speaker lids run off a Rockford Fosgate punch. I'll keep the trunk speakers stock as it comes off mostly. My system sounds pretty good.