If all you want is the excitment of going fast in a straight line,,,maybe drag racing is more your ticket.[which there is nothing wrong with]
That being said,,,having raced 125 GP bikes and 250's for about 30 years,,, anybody can go in a straight line. So a 125 CBR WILL
humble you and then you can be taught. At Turn 2 in the good days, I would ususally take an Aprilia 125 or Ninja 250 for half the day. While my
students were on 600's. Very few could keep up,,even in a straight line. But in reality,,most were happy to go slower cause they were over whelmed!
So understand the track is different. You don"t have stop signs or lights so you have to use your head and judge where to brake hard or just scrub speed,,, not to mention yes ,, you can drive on the left!. At Calabogie on a Ninja 250 I could do a 2:38[lap record is 2 minutes on a Szokes Superbike .Which means I was loosing 7.6 seconds per kilometer to a Superbike] NO straight line speed all corners! So when I got on a 6 or 10 the straight line speed just made it easier,,but you still need to manage corners.
explain why Rossi would take out a 125 GP bike anytime he could after "big bike" practice? Becasue it taught him to ride and showed his mistakes. Plus if he did get it a little loose or sideways it was more manageable
Big bikes will hide you mistakes,[until you hit the ground],, small bikes,,,wil slap you in the face every mistake you make. So be humbled and learn or,,, ride a big bike and use half throttle! There is a lot more to learn then just going to the track. Becasue you have riden on the street for X number of years,,,,really,,,trackside,,, means nothing!
A CBR 125 has a very short seat window,,,meaning you will look for something bigger if you master it. But what you learn will be monumentous. Not to mention,,,every year racers should rent/borrow a 125 and go humble themselves! Don't be afraid of a little bike!