Your stock exhaust may not be as lousy as mine was. It crippled all the gains from a modified engine. Really hurt mid and upper end. I'd say 5 hp difference for me with a header and straight through muffler. Our cams have some overlap so volumetric efficiency is sensitive to vacuum or pressure in the exhaust system. And that 14 hp carb is a limit too. I'd look at Whipper's header and maybe use the glasspack he found. I'd wouldn't use as big primary tubing as his though. I think a good exhaust and a rejetted one barrel would make you happy.
Now, forget about turning over 5 or 6000 rpm. Set your clutches up so it runs around 4000 as the cvt runs out. Our cams make good power at 3 or 4000. The compession is what pushes the pedals. Make sure you run good gas. Either AV gas or I'll tell you where to get a real octane booster, not the gimmick stuff at the parts house. A header and rejetting was a huge improvement for me.
I have my 18 hp rejetted carb I could sell.. have to ask George if the manifolds are the same on 14-18. Glad to hear it's running! We have stock flywheels, so I don't want to be near it at 7500 Also when George runs 8-10,000, it's on a couple hundred lb mower, with a $350 lightweight, ARC safety flywheel, and a modified 2 barrel with a bored venturi that flows 23% more. That's a $700 modified carb. And while he may turn those high rpms, that's just because his gearing was too low, and he pitted and re geared.
I think exhaust, carb, and fuel will get you what you want.
To Invent, you need a good imagination and a pile of junk. (Thomas Edison)