I was wondering if and how the damage would change if the quality of the Rune Pistols created by Gunhell would go up a notch, like say Quality +15.
Would it become 150 or stay at 100?

I am not sure how to deal with this, seeing as (by the rules) Gunhell pistols should only have a Base Damage of 80. I would say that the +10 to +15 Quality would add the usual +10 Damage. It would definitely NOT become 150.

that was my understanding as well...

but are you sure the guns are only base damage of 80? aren't you forgetting the strength of the pistols since if I recall each +5 of quality adds +1 to the strength of the base gun and if I'm not mistaken the strength of the base gun is 9 which gives it a +10 modifier, but since it's a quality 10 item then it adds +2 to the strength making the final strength of it 11 resulting in a +20 modifier to the damage... and by writing this I guess I answered my original question... sometimes one only needs a sounding board
thanks

That would be correct. I forgot to think about the STR mod.

A +5 Quality would definitely be a +10 Damage and +1Str (but 12 is no different from 11 on that side), hence +15 Gunhell Pistols would have 110Base Damage and +20 would have 125 Base Damage. Or so I'd rule it.

One moment - does the Strength of the gun actually apply to damage?

As they are written in the book, the Strength is already calculated. On weapons such as crossbows, you would need to add the weapons Str mod to the bolts damage.