A simple way to get the answers to your questions would be to find a Titleist Thursday event near you and book time for both a driver and iron fitting.

The basic performance won't change as much as you do over 7 years, unless the head develops a crack. The improvement in forgiveness is amazing. For that reason alone, many amateurs will average gains. Getting fit will likely get you 10 yards just because you probably have changed physically and the number of shafts available to fit just about anyone. Both your driver and irons will see improvement. Driver will likely be more dramatic, but you use irons more each round. Of course, you can't check the 718s for another 3-4 months.

I recently changed from the 915 to the 917. When I met with the fitter, he said up front that if we do not get at least a 10 yard improvement there is no us in changing. Needless to say we did improve by 11 yards, but the most important part was the accuracy. I had a 910 and moved up to the 915 because of the improvement factor. At 71 (well I will be there in 2.5 months) and I need all the help I can get. He had me hitting the ball consistently at the 240 mark which I figure is ok. Last week in a tournament I did bomb one 260. I am very happy with the set up that the fitter put me in, but the key is going to a Titleist certified fitter. Makes a world of difference. Now if I could just slow my swing down a little bit, it might go farther. My swing speed is 93mph, ball spin 2300, and ball speed is 133, but keeping those numbers consistent and under control is another factor I have to work on. Having playing softball for 26 years then taking up golf late in life makes me want to crank it way to often.

I'm the same as you, been playing with a 910 for years now but I'm losing confidence in it and most likely will benefit from the added forgiveness a new driver which is custom fit.
I am pretty confident that I'll be able to get extra distance but the biggest benefit I will be looking for will be accuracy.