No, you know that you can adjust turbo speeds under FIVR in TS, for the number of cores that are active, right? Say, for example, stock turbo bins are 40-40-37-37x for 1-2-3-4 cores active, you can dial down the turbo bins for higher core count load to something like 40-39-35-32x which I do on my XPS 13. That way, you get the high Turbo speeds on single- or two-core load to support your less intensive games, while dialing down the heat for all core load.
If you knock the 8750H down to its 2.2GHz base clock by disabling Turbo, i guarantee you're not having a great time. Especially when you have low graphics settings, which just makes it more CPU bound.
How much of an undervolt are you running? Also, are your fan speeds on XPS 15 able to be controlled via software, or are they hidden by the EC firmware as is on XPS 13? The EC is a pain in the ass because it has this stupid 30 second delay before it actually ramps up, meaning it can keep an optimized undervolted CPU under control, it just chooses not to until it's already throttling. Basically hysteresis, but idiotic and on crack.
It doesn't sound like the issue is not the long term limit. 15W is barely enough to sustain just 4 cores at 3.0GHz full load. That produces only enough heat to run completely passive, without throttling, through the 256M TS Bench.