Modern Buddy

Combined with up sized main jet, is 10% increase in performance a norm?
#92 jet is stock, up the jet to what #?
Besides reading the plug, what is another method to insure proper main jetting?

No one can tell you the correct size to use in your particular engine setup and location. I find correct main jet size the old fashioned way, by using plug chops.

If you're after more performance, there's plenty of aftermarket hopups available but keep in mind the PGO version of GY6 engine isn't always the same as generic China made versions. If you're looking at BBKs (Big Bore Kits) keep in mind that the 125cc engines use a 13mm piston wrist pin while the 150cc engines use a 15mm wrist pin. So, chose your BBK wisely. You can always go with a different crankshaft for a longer stroke and bigger wrist pin but you will lose the use of your kick starter unless you make modifications to the kick start mechanism. You will also lose the EGR system, if that matters to you.

Very good point about the shorter 150 cc Buddy stroke. I kitted my 06 125 years ago and it rocks!

I have never understood how a "high flow air filter" makes any difference without any other modifications.

You can't get more air in if you can't get more exhaust out.

The carb including jets, air filter and intake as well as the exhaust are the same on the 125 as the 150. The air filter is even the same as on the 170i and I think the exhaust might be the same on the 170i also. Since the aspiration/exhaust is the same on the larger engine(s) it sounds doubtful that you could add any significant performance with those upgrades on a 125. I did re-jet mine for a 161 big bore but it didn't take much larger jets before it was running way to rich (sea level to about 1,000ft).

No plans for a BBK.I do know when running at WOT on a 92 main jet and no air filter it ran very lean.
From the exhaust note with OEM air filter it may be a possibility that someone did the tried and true "rebar up the muffler" trick. Not much in the way of baffling.
A generic foam air cleaner is $8 incl shipping. OEM, $27+shipping.

I see the stock air filter element on eBay for $15, shipped.