As a guide for fuel consumption I've done 500km's on mine before filling it right up & got 22 & a half litres in it, so still had half a litre in it.
Roughly 100k's is 60 miles, so 500k's is around 300 miles. I ran with the F-Trip clocking up to 100k's to achieve those figures. So your consumption is a bit on the low side.
The fuel does swell if you park in the sun too. I've left home in morning in the dark with the F-Trip having just come on,( at around the low 20 degrees ) then leaving the bike till 3 or 4 in the afternoon ( temp's getting up in the high 20's ) finding the fuel level having risen to 2 bars before now.
I have also removed the snorkel, have Akra pipes, & fitted both Kev mod's.
All I could suggest really if you have done any tinkering ( apart from the snorkel ) if you can is to reverse it back fill up with fresh fuel & see what happens then?

With the gear changing, I find with mine now that just pulling in the clutch a small amount ( a quick sort of flick ) & backing off the throttle just a little instead of shutting off back to the throttle stop & letting the revs drop to idle when shifting have smoothed gear changes considerably.