A power supply does not throttle a graphics card or CPU. It either powers the system entirely, or it shuts down the system and possibly damages components.
Come on dude, you should know this.
This is actually not entirely true. While it is
usually the case that it either works, or it doesn't work at all, has instability, or randomly shuts down under load, there
are cases where a power supply can be defective in a way that leads to low GPU performance and "hitching" in benchmarks. I had this occur with a Radeon 7950 + Seasonic X750 combination several years ago. After swapping of components between two PCs I determined that the PSU was at fault. The RMA replacement restored normal performance (as did the PSU from the other PC).
This is extremely unlikely to be the case here, but it is worth correcting the assumption that such an occurrence is impossible (it isn't).