If the speed truly can't be matched it should just be close, however one last thing Shirou is already amped before the beginning of the fight since UBW is already active wouldn't Mega Man be brought down to shirou's amped speed ?
Also still in denial about the time stop ? In shirou's profile its litteraly stated:
"Limited
Resistance to
Magic, including [...],
Time Stop, [...] (All magi and characters with Magic Circuits have resistance to magical effects that aim to control and create effects within others, by rejecting the effect with magical energy).
Also when searching for 40 minutes i found this
thread. Where it's stated:
" Magic Resistance makes servants resistant to magecraft and that includes magecraft belonging to the emiya family which is able to slow down and speed up time."
Which contradict your claim that they don't get TIme Stop resistance from magic.
Funny thing it that in that same thread, it address your imaginary space argument but apply it to servants only never mentionning the magus. If you want to apply it to magus do a CRT but for now it's not valid.
Plus you haven't addressed the BFR at all.
The broken phantasm aren't stated anywhere close to a true name release, even by checking A
rcher's profile, yes Caladbog II and Hrunting are Low-B but because he release their true name in combination of being a broken phantasm. A broken phantasm simply increase the rank of the attack by one.
Also yes Gawain upscales from 962 gigatons, but Shirou downscales from Gawain.
Gawain (B+ STR)> Archer (D STR)> Shirou
Also let's accept that Shirou can keep up UBW for an infinite amount of time. Mega Man can still beat him with a lot of means like burning him with atomic fire or electrocute him.