I have taken a screenshot with my scope showing you a read-burst access.
![Image](http://atariforum.team-yankee.net/pics/caTTerror_ReadBurst.gif)
As you can see there is a pretty large gap between two read-burst transfers when the CPU is busy internally. So increasing read-burst speed more won't make a large difference - IMHO. Just increase cost (by factor 4) for a faster CPLD.