This is my understanding based on my research, so if I am wrong about anything hopefully someone else jumps in to correct.
It depends on which exhaust you get. many have silencers that can be inserted or removed as desired to regulate volume.
As far as numbers go, I only have manufacturer data, but akra reports a static db increase from 95 to 102 @ 5750 revs with their full system.
Link:
akra (full)
the akra link pipe allegedly moves dbs from 94.6 to 108.1 @ 5600 revs.
akra link
for a Graves EVR system,
Stock sound 80 DBA @ idle 91 @ 5000
Graves Cat Elim EVR (265mm) 86 DBA @ idle 98 @ 5000 (with sold separately sound insert) Graves Cat Elim EVR (265mm) 87 DBA @ idle 101 @ 5000 (No sound insert)
Link:
gravessome austin racing exhausts have controllable db levels and can range from 102 to 110+, but there is a power sacrifice at lower volumes.
Link:
austin
Most manufacturers don't provide volume data. The thing about the graves EVR system is there is a butterfly valve that you mentioned running on the exup motor. this valve is closed at lower revs, which has the benefit of 1) some volume reduction if that's not your thing and 2) backpressure to improve low end torque. the exup motor opens the valve at higher revs which gives you 1) a louder exhaust note and 2) higher flow to improve torque/hp higher in the rev range.
docs.google.com/spreadsheets/d/1Aez_XRww17osMrshIkeI_McLYl4gwHRnWnYM8Pbq4r8/edit?usp=sharing
this is a list of all exhausts i found when i was shopping. the links should take you to the MFG site so you can compare data.
there are two systems you're talking about here: AIS, which has to do with blockoff plates, and EXUP, which is the valve on the exhaust.
The AIS exists for emissions purposes. The system basically injects air into the exhaust system so fuel that makes it through the engine unburnt is given air in the exhaust system so it can combust. the environmental benefit is that unspent fuel is not released into the atmosphere. This usually happens in the catalytic converter, and when removing that you get weight reduction and a better flowing exhaust, but the drawback (if the ais is still running) is fuel blowing up in your midpipe which leads to a nasty popping sound that will piss off all your neighbors.
when you put an aftermarket pipe on, they recommend taking out the AIS and putting in block off plates to prevent that decel popping. the flash alone will disable the check engine that comes from removing the AIS, but the tuners i've spoken to recommend taking the whole system out and putting in blockoff plates to guarantee the pump does not continue to function. you can fabricate a plug (like the one graves gives you) if your flash doesn't address the AIS CEL.
this is independent from the butterfly valve that you are mentioning. That is the EXUP valve that sits in your exhaust. as stated earlier it regulates exhaust flow to optimize performance at varying rev ranges. Most full or cat delete systems remove the exup valve altogether as they're intended for track use, and on the track an exup valve is useless. Graves developed their EVR system that has a butterfly valve and uses the OEM exup servo. This is nice for street use because you don't sacrifice any power down low in order to get big gains higher in the rev range. I personally went with the EVR, but if you're on a budget it is not necessary.
i'm not qualified to speak to the power increase, but a flash is recommended by most riders even if an exhaust is not immediately installed, if only to smooth out the throttle response and remove factory power restrictions yamaha implemented on US models.
If you do go with a flash, can't recommend 2wdw enough. They are about as good as it gets when it comes to a business, and I'm sure they can give you more info and explain it more clearly than I can.