You wouldn't even get into top gear, let alone pull 110mph.
Check the R3/Ninja300/cbr300 dyno graph, none of those engines are happy below 5000rpm.
Best to buy a pcx125 scooter with an automatic and get 100mpg.and save the $3 per week for a coffee.
If the bike can do 116MPH in top gear, then it can do the same when you make 5th or 4th gear ratios almost identical to stock 6th gear ratio, and have 1 or 2 overdrives.
Most of the time, ptwins work ok above 3k rpm (most of them below 1500-2500RPM, they would stutter, or hesitate), and I read (but don't own the bike), that it start pulling good from 5k rpm upward.
So I understand when you say that you want to stay in those 5+k rev-ranges.
That still will be possible with a gear change!
Just not in 6th gear; but perhaps in 5th, or 4th gear.
A 125cc's top speed is around 65MPH, not 110, so while you get the MPG, you don't get the top speed.
Gearing mods can improve both top speed and MPG through choosing the right gearing;
It can:
AND get you 85+MPG (@low speeds, low RPM in final gear; eg: doing 40-45MPH @ 3-3,5k rpm in 6th)
AND get you 110+MPH (@lower gears, higher RPMs; eg: doing 110MPH @ 11k rpm in 4th gear instead of 6th).
There's only one con to this, other than having to buy new sprockets to install (and sometimes a chain as well):
Acceleration from a stand still to the powerband in first gear is slower.
All other acceleration (from 1st gear powerband onwards) to top speed should be very similar, if not faster than stock gears, (since you'd have to shift 1 or 2 gears less to get to top speed).
It all depends what kind of gearing changes are chosen for the bike.
Too much one way, or the other, is never good.