From what I have heard it's not really adviseable to go much more than 14 inches of depression with the Vac pump, two reasons
1) the trade off at anything much more becomes negligable
2) it will have to be well sealed as at much more you may start to suck air past seals etc..
this was on an Austrailian touring car engine, small block ford, about 600 hp and 8000rpm (similar to nascar)