I always run with 300-400ms latency. Of course, in this situation I'm the healer and have a paladin-specific strategy that essentially makes it impossible for me to die as long as the tank is still up.
I ran Naxx 25 up until Thaddius last night. 2 things.
1. My connection is fucking terrible. Every ability that affected a large number of people had a good shot of disconnecting me which made me wonder why I never got booted from the group (I said in raid that they should replace me multiple times). Thane's Meteor, DC. Gluth's Decimate, DC. Etc. On Thaddius I DC'd on polarity shift once but got the same charge so it didn't kill anyone. When I logged back in I ran away from the raid and continued to heal from 40 yards out. This really hurt everyone's dps, though, and after a <100K wipe I excused myself.
2. Naxx 25 is awful. It's just one big DPS and AoE heal zerg. Naxx 10 isn't difficult but with two healers it's engaging and fun. This was not. If it wasn't for the ilvl 213 loot (really? 13 item levels above the 10 man?) I wouldn't go back.
edit:
My haul from last night -
http://www.wowhead.com/?item=40625 - T7.5 Chest (I'll get the Ret chest since the Holy chest sucks)
http://www.wowhead.com/?item=40204 - Legguards of the Apostle (perfectly itemized Holy legs)
http://www.wowhead.com/?item=40259 - Waistguard of Divine Grace (perfectly itemized Holy belt)