90s can manage to keep to 91 timings on Leeds services since they are faster from a stand than 91s. Whilst 91s have a high continuous rating, the maximum power is quite a bit less than a 90 (or 87 or 89), plus they are higher geared. With Leeds services having quite a lot of stops, 90s are well suited (they would loose out if used on limited stop services, although once north of Newcastle they would likely gain time back due to the lower speed limits). I'm suprised GNER didn't go for 90s (or 87s, which can pretty much match a 90, having almost identical output, although having a slight handicap from the tap changer), since they should be much better than HSTs on that sort of run, having much better performance (especially with 9 coaches). I suppose they didn't want to take on another fleet though, especially since 90s don't have that much in common with 91s, and 87s even less. I do wonder if there are power supply issues aswell.
Regarding the tilt on the Mk4s, I don't think this was ever intended for use on the ECML. The 225s were originally planned for use on the WCML, with tilt, as a simplified production APT (the 91 is infact heavily based on an APT powercar), with the ECML having 89s and Mk3s (probably with power operated doors like the 442s). As it happened, the 91s went to the East Coast, and the WCML got 90s to replace the 1st generation electrics, DVTs, and with 110mph running for 90s and 87s. I don't even know if the Mk4s had tilt equipment fitted. As has been said, the space on the Mk4s for tilt equipment was taken up at refurbishment anyway. Also, the 91/1s are restricted to 125mph, so even 140mph seems unlikely.