Joey,
If you're using a VPI(for example) then the Achromat will be doing you no favours at all, you're better off with a bare platter.
The only thing I can say in the Achromat's favour is that it has a slightly lower noise floor than all other mats I've tried (damping effect), everything else (IMO) is downhill but ultimately it can boil down to a matter of personal preference.
One thing's for sure, if the LP has fresh air underneath then the Achromat won't be doing the job it claims to do (you could extend this disadvantage to the use of tape patches underneath the Achromat for self-adhesion to platter. These would also marginally change the behaviour in the same way i.e. decoupling).
No LP is truly flat though so you are bound to see this kind of decoupling randomly happening whether the LP is dished or warped ;^)
