64 bits memory doesn't do very much in this case.
in this case, this is not specially doom. and this is not specially graphic card too.
this is simply the effect of the 060 wasting ton of clock cycles to read/write in memory when the bus is divided by 2... it's not very hard to understand.
it's a bit like if you get a car engine from 72 to 100 HP, but divided the gas pedal run by half. (i don't know if "run" is the good word in this context but english is not my native language)
look : here is my Mk2 at 66 Mhz 1:1 bus :
https://www.youtube.com/watch?v=nPKMsKGj_1gcompare here at 100 Mhz :
https://youtu.be/9zCHGWytf9Q?t=73what do you see ? and please watch carefully

i have a very little bit better framerate at 66 Mhz... why ?
because at 66 Mhz, the bus is synchronous, and the 060 despite the fact there's less raw mips... waste a lot less clock cycles to access memory.