a free flow exhaust and a stock replacement filter should be more than enough for your regular use..
since you are from mumbai..automech is the place to go for exhausts...no one in mumbai makes better exhausts...and i am sure many amongst us will vouch for that ..
about the upsize read this ,
http://www.team-bhp.com/forum/showth...ghlight=upsize (The right tyre upsize)
and for better handling you can geta strut brace ...its cheap ...will cost you about 1.5k at the most ...and you can get all this from automech..you can search for a lot of topics on strut braces on this forum ..
beware of people making tall claims !!!!!!!
and,i suggest you to make all your modifications in steps so that you can understand the difference..
whatever mods you plan to do ...do keep us posted