Are there any GFX cards currently available which accelerate 422? Don't think so - 422 decode will be done entirely in software, so a CPU with significant grunt is required.
Of course, the NVidia GFX is useful to have for RBT, but will be ignored for 422 whether H264 or H265. So CPU, CPU, CPU! in the budget spend.
The tuner itself is agnostic about the video codec... it's just data / ts packets it sees. So, if the tuner can lock the transponder, it will pass on for decryption & decoding all packets irrespective of whether those ts packets contain H262/4/5 420/422 audio/video/teletext/whatever.