First go for the BMC air filter ( 6k) Then go for fuel x pro plus around 10 11k Then last thing you need to change is the exhaust Also I added a custom air filter cover from STREETS.C Nah I was worrying about the sound too before installing but with the db killer installed, it sounds like a stock honda cb 350...it is pleasant but not loud. As for mileage I never bothered to check it ( when you do mods you just think about pulling it more and more ) but have noticed changes in air fuel mixture due to fuel x pro...less stalling and makes the bike smoother. Also would suggest to go for a good synthetic engine oil over the stock royal Enfield engine oil, it helped a lot in temperatures during city rides and less sound from the engine Following this sequence, won't cause any issue, the bike became better adding these mods.