Steam Deck has up to 1.6GHz TFLOPs.
That means up to 1.5625MHz clockspeed for the 8CU IGP.
Switch has supposedly 6x more Cuda than the previous version. Link
That’s 1536 cores, If It’s Ampere based, It would mean 12SM.
For It to be on par with Steam Deck It would need only ~800MHz.
If It has only 8SM, then ~1.2GHz.
I personally think Nintendo will have to increase TDP to 15W, but It will be compensated by a bigger battery.