Well it's all gonna depend on the type of claim required, ie, if your boot handle falls off while driving along the road does the fact you fitted a boost gauge invalidate your warranty?? obviously not.
Now then, if your dash catches fire..........see where i'm coming from????
I cannot see how any reputable garage could not honour warranty claims on each claims merit.
If you are unsure, do what i did, get the garage to buy into your thinking and upgrade.
My garage were superb, although i have not got anything in writing i feel pretty comfortable about my warranty and most importantly my TLC.
Go for it, i'm looking into the BMP gauges at the moment awaitng a response.
Hope that helps
Alan fae bonnie scotland