A pipe that requires or suggests larger jets is doing so because it has a better flow rate, and/or less back pressure.
When the piston pushes out the spent gases it is partly restricted by that back pressure, and cannot completely expel everything. Less back pressure equals better "scavenging". Therefore the jetting is set to accommodate this. (Rule of thumb... The more gas/air you can get in the jug the more power you can make.)
Sooo your air fuel ratio will remain the same coming in but the ratio inside the jug will be leaner because there is less of the "spent" mix remaining which does contain some unburnt fuel.
This is on a VERY small level, and again it may be made up for by enriching the idle circuit thereby enriching the entire mix ratio.
Like I said this is a very small scale, and most pipe makers simply suggest up jetting to increase performance. That is the primary goal for most people changing pipes. Without up jetting or enriching the mix you may actually decrease performance. There is no way to know unless someone that used the same pipe on the same engine can tell their story.
My personal opinion is to go ahead and try the pipe you like, and see. The worst that could happen is you may indeed need to up jet, and that is SOOOOO easy you will wonder (in hind sight) why you were even concerned about it. No dealer on EARTH could ever tell or have any way of knowing the jet was changed because so many different jets find their way into the same engines there is no way to verify what is in what.
In thinking about it a second longer you (again) may not even have to go that far. There is a main jet "needle" that meters the main by sliding up/down inside the vapor tube. That needle is adjustable, and can be raised to allow more fuel thru the same jet sooner thereby enriching the circuit. That along with the idle mix should get you back into the "sweet zone".
Don't over think this... It is simple to do.
