In my BIOS I can set the processor speed in a list. The maximum speed in this list is "1400 (133)"
However, there is also a "User Defined" option in this list where you can select the multiplier and FSB etc. When I set this to "User Defined" the highest multiplier available is "12" but there is also a "12 plus".... and I'm not sure what "12 plus" means.
12 x 133 FSB = 1596 MHz. I suppose I can only try the thing out. If it doesn't work then it doesn't work..... c'est la vie!
I'll do some benchmarkings on the 800 first to see what improvements there are overall (if it works). I suppose this is all a bit overkill for a Mame only cab system.....

...... but I do find it interesting and it would be a real bargain for the 1800 if it works.