I have a water shield on mine and on really wet days, sometimes you need to twist the throttle at startup to get it to start. This means I have to get it on the kickstand to start it.
Also, on the highway, also if it's really wet, sometimes it'd cut momentarily when I give it enough throttle to jump to high rpm's around 7000. If I throttle up gently, it wouldn't be as bad but if I twist the throttle hard, the cut is very noticeable.
I'm about to change plugs and that would be telling if I'm running lean overall. The noise of the induction is also much louder...rivaling my usually louder mufflers.
The only reason I'd revert to the OEM box though, is if at the upcoming plug change, I discover that it runs excessively lean. I can say that from my observations, my exhaust runs hot, so that could be one sign. I've been running with the cone filter mod for a few yrs at least with around 14,000km.
This is the thread when I had assembled the mod:
https://modernvespa.com/forum/post1465906?highlight=#1465906