I am using a TG4 now in nauticam housing with 2 strobes for wide angle. You can check out the kind of photos you can squeeze out of this little camera on my instagram, search for speedyredfins. I was curious if TG5 is any better than TG4, but after discussion with some photographer friends we came to a conclusion that since the sensor size is the same, it would be very similar end product, esp if you are using strobes where low light is not an issue. For me, my nauticam housing cost me $900 so to shell out that amount of money for some marginal upgrade does not make a lot of sense. But hope one day to borrow a TG5 to really compare the quality.
I'm currently using a similar setup with a tg-4 except I have the olympus housing. I had similar thoughts when the tg-5 specs were released; the sensor is the same tiny sensor so it will probably still be the biggest limitation (other than not full manual). I am curious what exactly "low light performance" means, what was adjusted, software in the camera? Although, with the reduced resolution there is more info per pixel, perhaps that's where the low light improvement comes from.
These tough cameras can take some pretty good pictures, especially with strobes, but any upgrade I do from the tg-4 will involve two things, full manual and a bigger sensor which the tg-5 doesn't offer.