Even in portable mode, Switch's GPU is better than PS3's. If we look at games they share, like FIFA 18 and Skyrim, Switch performs better both docked and undocked.
Switch ports of PS4/Xbone games wouldn't have to be cut back as much as Wii ports from PS3, because the power gap is much smaller in the former case.
Switch has weaker GPU in undocked mode in terms of raw power than PS3. It has some advantages like modern architecture, shaders and all but still we are talking about PS4 and Xbox one games here. The hardware power difference is big even in docked mode. Like i said Doom shows how big it really is in practice. Doom is also one of the best looking and running games on PS4 so think how bad a game like MHW will look that runs at a much lower framerate on PS4.
I know Switch is more comparable to PS4 than Wii was to PS3 but the difference is still really big.
I sort of responded to this before (in a reply above), but I will paste what I said in that post below (just as a summary, the GFLOPS number is not in many cases the best indicator of real-life performance; in practice the Switch's GPU should be equivalant or more powerful than the GPU in the PS3 even when not docked, but of course nowhere near as powerful as what is in the PS4 and Xbox One). Here is my argument pasted below again:
"As for the GPU, I am assuming you are looking at the floating point operations per second (FLOPS), in which case yes the PS3 is theoretically faster than the Switch in portable mode. However, that number may be deceiving because there are many other factors that need to be taken into account, and in practice the Switch in portable mode should be, at worst, on-par or, most likely, even faster than the PS3 GPU. Here is a good example as to why GFLOPS are not the end-all, especially in gaming performance. This link (https://www.tweaktown.com/tweakipedia/116/fury-vs-gtx-1070-battlefield-dx11-dx12/index.html) benchmarks the AMD Radeon Fury X (8601 GFLOPS) compared to an Nvidia GTX 1070 (5783-6463). Going by the GFLOPS alone, the Fury X should be able to handedly beat the GTX 1070, but in most cases the 1070 outperforms the Fury X. Now it should be noted that Fury X and GTX 1070 were released in 10 month period from one another; there is a 10 year gap between the Maxwell-based GPU in the Switch's X1 and the PS3's GeForce 7-based GPU, so even while the theoretical floating point is slightly higher on the PS3 than Switch in portable, there are numerous other advantages that the Maxwell GPU has over its PS3 predecessor that should give Switch in portable mode the performance advantage (even if it is a narrow advantage)."