Your math is off.
Vanilla patchwerk: 3850000
WoTLK patchwerk10: 4322950
WoTLK patchwerk25: 13038575
Vanilla average top DPS for a guild in gear from BWL and AQ40: 750
Vanilla dps per raid: 26
WoTLK average top DPS in the start of expansion gear: 2000
WoTLK average top DPS in the Naxx10 gear: 4000
WoTLK dps in raid10: 6
WoTLK dps in raid25: 16
Assuming same skill
P10 DPS ratio adjusted for dps per raid (start of the expansion): (6/26) * (2000 / 750) = 0.6154 (tanks unaccounted for)
P25 DPS ratio adjusted for dps per raid (start of the expansion): (16/26) * (2000 / 750) = 1.641 (tanks unaccounted for)
DPS ratio adjusted for dps per raid (after Naxx10): (16/26) * (4000 / 750) = 3.282 (tanks unaccounted for)
Patchwerk10 HP ratio = 4322950/3850000 = 1.1228
Patchwerk25 HP ratio = 13038575/3850000 = 3.3866
Conclusion:
if you went to Naxx10 in questing gear/blues (no heroics) - you would be having a hard time in there.
if you went to Naxx25 before going thru Naxx10 - you would be raped in there.
if you went thru Naxx10 (as you were supposed to) first, you would be experiencing the same difficulty as in Vanilla (excluding the necessary changes to mechanics for WoTLK class changes) BUT you would know all the tactics.