For what it's worth, I have a 997.1 4S and for the longest time I kept asking myself "Where's the beef?".

I subsequently put in a IPD plenum, racing headers and 200 cell cats. The car became spirited, alive and considerably more fun to drive.

 

To the OP.

You've already heard 991 is a different car. So it's an apples or oranges discussion. You like apples? get the apple.

One other point you have with a modded 997 is that it is customized to your liking. Having a new stock 991 is great, but your neighbor could buy one just like it. Whereas the customization (with zero or little payback) allows you to tailor it to just you. IF you are into that sort of thing...

We could get into a modded 991 discussion, but then we are going beyond apples and oranges and talking bananas!

 

In this situation, I'd save up and get a 991. It'll cost a lot of money to squeeze a few HP out of the car. Unless you put on a blower.

The 991 is a beautiful car. Love the new interior.

 

A generalization but, if you're looking at different car, no mods you perform to your existing will take a way that urge to switch. You'll just end up spending (and not recovering) money that could be put towards the new purchase.