I have no hard data, but based on some "in-between" updates (maybe Hellbound? It was some update close to it) magical critical chance was modified by 0.33, so 0.35 seems like a real possibility (even tho this is regular damage and not critical chance) - I mean, they tended to always use the same values based on their language, like greater used to mean X% etc.
I've tried to search for some l2w logs that I had but I didn't back'em up (and on Epilogue I had only limited data as I ran a 5man party, not multiple like GF).
The numbers seem good; being able to fine tune easily in the .ini is awesome. I'd feel tempted to use the damage nerf at about 0.2 to 0.35 (test what makes solo chars struggle to farm the new mobs). I hate to guess =/
edit:
I found this post: http://boards.lineage2.com/showthread.php?p=2941563
"If you are lvl 80, trying to attack a lvl 84 Megacrokian on the Field of Whispers, you will deliver less damage then you would on a lvl 81 monster of the same M.Def. If your Wolf in that example is lvl 82, i.e. 2 levels below the Megacrokian, you will also notice a reduction in the damage it does. That damage reduction is calculated separately and independently from your damage reduction. You do a lot less damage with your nukes (in Taiwan there was somebody who tried exactly that and needed 10 instead of 6~7 shots) and your Wolf does a bit less damage with its bites. Since the reductions are adding up, this means that the Field of Whispers is de facto not doable for you."
35% reduction seems pretty legit.
edit2:
"In Korea the level difference nerf was first not there in Gracia Epilogue, then it was introduced, then it was removed again, then it was re-introduced. It seems that technically it would be no major problem to add or remove this feature from the game. Maybe we should give it a few months on the live servers, see how it works out, and then make a poll."
I don't know if this si true, but it sounds legit based on NCs past behaviour. It seems likely they added this nerf only because of their high number of players - to keep the new areas more exclusive.