hi,
It's probably very near to the limit, but it always worked.

I know that way. Well, it works for you. Some of my friends who use the same way, report me that sometimes signs on LCD are not printed randomly. When they decrease OSC frequence the problem gone out.
So, as I have already said: read LCD datasheet - it contains all timings described. Do not try to overclock any hardware, it may cause undefined results.

By the way, "modern" derivatives have bit(s) to control Stretch of /WR and /RD during MOVX. It is usefull feature for such purposes when you work with slow peripheral devices.

Regards,
Oleg