However, I do wonder if you suffer from many of those outlined issues so you can maybe indulge me.
1) Fighters have a strong secondary Striker tendency and can be flat out built as alpha Strikers. If neither of your Fighters are Sword and Board (even those can be built as strong secondary strikers) and/or if either are built to be alpha Strikers.
2) You have no pure Leader and an abundance of primary Strikers, secondary Strikers and a Wizard (which in a game where there are as many enemies as you would have in the encounter budget expectations of 7 + PCs, they would easily be Strikers given their potential DPR).
3) I doubt, given your experience with the ruleset, that you guys bog down at all in the time:turn ratio.
So, I'm not sure you suffer from two of the main problems; too many pure defenders and too many pure leaders with no synergy - eg Lazy-lord + Slayer/Barbarian or Radiant Mafia. If you build a Radiant Mafia + a group underwritten by Lazy-lord + Slayer synergy then the efficiency should go up with numbers rather than down. Secondarily, see 3 above. Without those 2 main issues in play, the wieldiness difference of 7 PCs, vs 4-5, is likely marginal at best. You can correct me if I'm wrong about any of that. Again, your experience in this domain is more than my own (considerable:0). I'm just Thomas Jefferson building attempting to build a plow on mechanical extrapolation/deduction with no practical experience.