I have been struggling with this problem for about two years now, I have an Acer swift 3 (I5-8265u, MX250). I just saw this thread while trying to find a solution better than the one I've been using for about a year or so. The only way to keep GPU Tweak's modified maximum temperature as far as I have found is to use MSI Afterburner's "Restore settings after suspended mode" feature to save it's settings (Which is the undervolt profile) and GPU Tweak's modification at the same time after a shutdown.
Usually Afterburner and GPU Tweak override each other, but if you shutdown your laptop after applying GPU tweak's modifications then afterburner's profile (in the exact order) and then boot it up again, your laptop will boot up with the undervolt profile and the modified max temp loaded and will stay there as long as you do not use the restart button or you do not do anything that modifies the GPU driver or shuts the GPU down. For this to work you need fast start up to be enabled, and this will work regardless of the bios version or the driver's version. I have been using it for quiet some time now.
All you need to do is the following:
1- Enable fast start up if it wasn't enabled.
2- Apply GPU Tweak's new max temperature limit.
3- Enable MSI Afterburner's "Restore settings after suspended mode" feature in the settings.
4- Apply Afterburner's undervolt profile.
5- Shutdown your laptop normally.
6- Boot up the laptop.
7- Voila, try not to burn your laptop.
Actions that used to undo the modifications:
-A restart instead of a shutdown.
-GPU driver update.
-Windows update.
-GPU being disabled for some reason.
I was able to keep the modifications for about two weeks without reapplying them, so it's quiet effective. Checking if the profiles are working is easy, just open MSI Afterburner and check if your undervolt is there, if not, then your max temp is back to default too.
My GPU was capped to 75C, so i started my quest by:
1- Undervolting and underclocking the CPU (had to undervolt it by 100mV, and cap it's clocks to 2.2GHz to have reasonable 80-90C temps), using Throttlestop.
2- Undervolted and underclocked the GPU (had to undervolt it by about 75mV, and cap it's clocks to 1420MHz to have 75-83C temps), using MSI Afterburner's curve editor.
3- Set the max temperature limit of the GPU to 83C.
4- Repasted everything with Arctic MX-4.
5- Put thermal pads between the one and only heatsink and the bottom metal cover.
6- Have an external cooling pad concentrate the air flow to the area where the thermal pads are.
7- And now I'm confident that buying a 14 inch ultrabook with a discrete GPU was a mistake.
I was even able to play detroit become human at 720p 30fps. I usually play Civilization 6 at 1080p constant 60fps on huge maps, I even reached 120fps at 900p on smaller maps, destiny 2 used to 900p 60fps but had to go 720p after some updates, COD: Warzone at 720p 40fps wasn't playable for me so i dropped it, all at low settings and constant framerates with no spikes. The GPU usually hits the 83C limit but no actual throttling spotted while gaming. Before all of this, spikes where all over the place and even CS GO was not playable. But after all, even though my MX250 version was supposed to be 25W, after all of these modifications I'm barely hitting 10W, so why did Acer decide to put a 25W GPU into a laptop that can't handle a 10W GPU?
I never thought of buying this laptop for gaming, I'm a student that needed a portable machine for a good price, but not being able to game made me want to be able game, and the above modifications even helped me alot at doing some 1080p video editting on this thing, because the CPU mistakenly benefited the most from all of this. I did all of this because I like tinkering and messing with things, so whether it's worth it or not is up to you, it was for me.
EDIT 1: Below is the GPU voltage/frequency curve in Afterburner, might help you get an idea of the modifications I did in Afterburner.
and I actually managed to have decent FPS and almost no throttling at all by undervolting / underclocking the MX150 at 810Mhz/712V.
