Are you STR or VIT stacked?
I based the "20-25% increase" figure I tend to throw about on a combination of the extra STR, Crit, Det and SS you gain going from a "BIS VIT/PARRY" build to a "BIS VIT/STR/DEX" build and the effect that has on the DPS output of a PLD that's wielding a Curtana +1.
The spreadsheet I have to hand on this PC is an older copy and I've since tweaked the builds slightly, but it'll give you a rough idea of the stat changes:
Det with full Gryphon Gear: 292
Det with full Hero/Allagan: 230
Crit with full Gryphon Gear: 368
Crit with full Hero/Allagan: 352
SS with full Gryphon Gear: 359 = 2.156s GCD
SS with full Hero/Allagan: 352 = 2.162s GCD
Plugging those values into http://valk.dancing-mad.com/calculat...ut-calculator/ (though I notice now when I link this that the calculator page has been altered recently because the old URL I had doesn't work) gave me a rough difference of:
+18.30% (Crit-Adjusted WS damage)
+26.83% (Crit Adjusted AA damage)
Because of the enmity modifiers on PLD WS (RoH combo!), I figure that it's going to be a lot closer to the WS value rather than the AA one.
[EDIT]Whilst cleaning up my more up-to-date sheet (see below) I've redone this calculation. Moving from a BIS +VIT to a BIS Gryphonskin build appears to grant just over a 14% increase in WS DPS if you try to keep Parry as high as possible with DPS being a secondary concern, and just under a 17% increase if you try to keep DPS as high as possible with Parry being a secondary concern. HP for both these Gryphonskin setups is 6694.50 HP, compared to the +VIT build's 7057 HP, a decrease of 5.14%.[/EDIT]
Me? It's more a personal equipment planner for my own PLD than a user-friendly calculator I'm afraid... but I'll see if I can dig out the updated copy tomorrow and export it to Google Docs
[EDIT] Couldn't convert it to Google Docs, too many formatting differences. Have cleaned it up a little and uploaded it here instead (it's a Standard *.xls file with no macros) - the link should stay valid for ~30 days.[/EDIT]