Re: Dell recommends a maximum of 50W PCIe power draw for the SFF models. no worries.  that 50W max is not written in stone.  Dell SFF user has frequently installed 75W gpu such as GTX 1050Ti in 50W max slot and run the video card successfully. you also have option to upgrade the 7090 SFF psu to one that has PCIe black connector to support more powerful gpu that needs 6 pin.  7090 SFF psu is interchangeable with 7080/7090 tower psu in form factor and motherboard connectors which all have the white 6+4+4 pin connectors for motherboard.  the Dell config does not have the high power psu for SFF is just Dell's way of limiting SFF.  does not mean user cannot. Tower:260 W typical 85% Efficient PSU, 80 Plus Bronze300 W typical 92% Efficient PSU, 80 Plus Platinum360 W typical 92% Efficient PSU, 80 Plus Platinum500 W typical 92% Efficient PSU, 80 Plus PlatinumSmall Form Factor:300 W typical 92% Efficient PSU, 80 Plus Platinum200 W typical 85% Efficient PSU, 80 Plus Bronze 
I would like to upgrade the graphic card in my Dell Optiplex 7090 SFF. It has a 300Watt PSU. Currently has a AMD Radeon 540 1GB LP graphics card. It has internal slots for PCIe Expansion I understand that the safest option is to upgrade to either GeForce GT 1030 2 GB GDDR5 Low Profile (30Watt) or AMD RX640 (50Watt) as Dell recommends a maximum of 50W PCIe power draw for the SFF models. However, I wanted to check whether the newly released AMD RX6400 low profile might also be a suitable alternative? AMD has branded this new model with a total board power rating of just 53 watts. 
Upgrade the graphic card in a Dell Optiplex 7090 SFF