I would suggest that you compare US Attainable rate (MAX) & sync speed (Path or Bearer) from before & after the swap.
These are my current stats (Mk2 SSFP):-
Max: Upstream rate = 4121 Kbps, Downstream rate = 21244 Kbps
Bearer: 0, Upstream rate = 4055 Kbps, Downstream rate = 21247 Kbps
They didn't change much at all when I switched from the Mk1 to the Mk2 faceplate.
Also, you could look at Hlog, SNR & Bitloading graphs to see if previously usable tones are now unusable.
To confirm if the different SSFPs do affect performance etc., you could try swapping back to the Mk1 SSFP to see if the missing data returns & then back to the Mk3 SSFP to see if they disaappear & affect other stats again.