IRQ timer is from the same XTAL as the main CPU clock. For correct timing, they would either have to have 2 XTALs, or a ROM modification.

Here's where I saw the mention of the timing difference: https://tinyurl.com/y3k3jfpg
"By the way, when using the 540+, be sure to set the ‘B’ time limits for 25% to 38% longer
than you really want, as the time limits have not been re-programmed for the faster speed."