> I was taught that you want, usually, more threads per block > than each SM can execute, because SMs context switch between > threads (fancy hardware multi threading!) on memory read > stalls to achieve super high throughput.

You were taught wrong...

First, "execution" on an SM is a complex pipelined thing, like on a CPU core (except without branching). If you mean instruction issues, an SM can up to issue up to 4 instructions, one for each of 4 warps per cycle (on NVIDIA hardware for the last 10 years). But - there is no such thing as an SM "context switch between threads".

Sometimes, more than 432 = 128 threads is a good idea. Sometimes, it's a bad idea. This depends on things like:

Amount of shared memory used per warp

* Makeup of the instructions to be executed

* Register pressure, like you mentioned (because once you exceed 256 threads per block, the number of registers available per thread starts to decrease).