You're close to maximum-timing anyway. Similar mod is to install '88-94 pre-gen ignitor. Which has 7-degrees more ignition than '08-12 new-gen. More won't give you much even if you bump up octane to 117. Full exhaust will give you +22% +6bhp. About as fast as 300. Even better to increase compression and displacement. Way, way more gains than anything else. People have gotten
double factory power this way.
Here's chart of bone-stock 250 with snorkel removed. It's got way, way too much fuel anyway. Your carbs are most likely dirty and clogged. Need complete carb disassembly and restoration... And get a wideband, reading tea-leaves gets expensive with buying new plugs for each run.