To add to my above report about performance with these additional rooms and open hatches, I have consulted various good links about how and when to cap FPS, and wether to use VSynch=ON in the game settings, this also to run at best the FOtRSU mod package. Here my conclusions:
a)
Set Vsynch=ON in the game.
If you have a 60 Hz monitor you will get 60 FPS with a larger frame delay and latency, but the good is: With any reasonable GPU the FPS will not drop much for you to see a real difference, and you will get rid of screen tearing and input lags. Also good is that your GPU load will be reduced compared to illimited FPS and Vsynch OFF.
b)
Set Vynch=OFF in the game, but...
First run the game with a tool that tells you the FPS onscreen. If you see a very large number like 500+ FPS while in your sub and moving, then you can safely set a framecap of 2x your monitor refresh rate, say 120 if you have a 60 Hz monitor. With that, if you don't get tearing or imput lag, great
Very good is: Even if the FPS drop during game, you will never see or notice this at such high FPS.
From there, measure your GPU load, if it's just below 50% with a good GPU you are just fine, if it's around 20-30% you can raise the cap, the more FPS the better until you get around 40%-60% load and your GPU temperature doesn't get too high.
If instead you get tearing after capping your FPS at 2x your monitor Hz, then you need to raise the cap until it goes away while still not exceeding 50-60% GPU load. With a lower card this may not be possible, so better go with a) and set Vsynch=ON, also if your GPU load is around 70-80+% which is not necessary for this older game.
I hope this little excursion in the FPS world helps!
XS