Going to 15T is the cheapest and hassle free solution. You can even do the sprocket change yourself if you have time it is pretty simple really. And if you ride more on highways then you will like this setup as you can happily cruise at higher speeds & the bike will feel a little more relaxed.

Holder plate and nuts should be available at Bajaj spares. But I had to run around a few shops to find it.

Guess I will try 15t first..if too tall will go for 44t set up.
The holder plate and nuts are easily and separately available at ASC of bajaj,right?

@albyjose, The 15T front sprocket for P180 costs less than 100 bucks (genuine one) but you also have to find the holder plate and the nuts too. The existing one cannot be used for 15T.

44T rear sprocket should be a direct fit since same rear wheel setup is used in both 160 & 180. I am not sure of exact cost of 44T but the entire chain sprocket set costs a little over 1000, so I am guessing it would cost around 400-500.

I guess you mean with modified 15T setup it looses its fun character? Then it is partly true. On RTR180 with stock tyre (65kmph at 5000 rpm), if you plonk a 15T at front it changes to 69-70kmph and the reduction in acceleration is quite noticeable, but since you say your bike gearing is shorter than stock (due to r15 tyre) then the reduction in acceleration maynot be as much & the bike might do 66-67kmph at 5k rpm.

There is another option of changing the rear sprocket to that of RTR160's. 180 comes with 46T rear & 160 comes with 44T. In simple terms, one tooth change in front sprocket teeth is equivalent to 3 teeth change in rear sprocket. So clearly, shifting to RTR160's rear sprocket might give you similar performance as stock RTR180.

Hope this answers your question.