Yeah, 4/5 Exenterator would definitely have an impact, though I still wouldn't expect such a large difference as what he saw.
Putting together a theoretical set. Conditions:
1) No more than 5 gear swaps for a weaponskill (ie: what you can fit into a single normal macro; no Spellcast)
2) Main TP set wants a decent amount of evasion
Target evasion: A naked thf has within around 10-20 evasion (5%-10%) needed to cap evasion rate on DC Dynamis mobs, and is possibly entirely capped. More than about 50 evasion in gear is unlikely to be useful against any NMs you might consider soloing, so avoid excessive evasion beyond that.
Took your gear set from the other thread:
31 Agi/85 evasion [100 total]
40 Dex/13% crit rate/14 acc
14 Str/14 att
0% Haste/2% DA/1% TA
Tried to keep things reasonably accessible. Allowed Sigyn feet since they're pretty easy to get (only take 10 Umbrage plans). Thokcha is Agi. Other things are AH-buyable or simple Abyssea drops or normal quest rewards.
55 Agi/55 evasion [82 total]
48 Dex/5% crit rate/15 acc
14 Str/20 att
26% Haste/13% DA/3% TA
Drops 18 evasion compared to your set, but you'll get that back with Exenterator. Athos body would be an improvement on the Dex/Agi/Evasion front, but surprisingly the Store TP on Loki's may be worth more for total damage. Overall, not worth requiring a piece that may be a pain to get, but it's an option to consider for an improvement on total evasion.
Also considered Vassal back for a bit of extra evasion, but it didn't seem worth it compared to a piece that you'll already have for other jobs. So, for inventory savings, just went with Atheling. Still an option if you want it, though.
Gains 8 dex, but drops 8% explicit crit rate. This is a small loss on my set's side.
Same Str, minor gains in acc and att.
Gains 11% DA, 2% TA, and goes from no haste to fully capped gear haste. This line alone is about a 45% increase in damage output (more if you have outside haste buffs).
Only 3 slots are really open for substantial changes for weaponskills: neck, waist, legs. Just going to put in a bunch of attack gear here: Justiciar neck, Anguinus belt, Tumbler Trunks.
On the DC target, that puts Evis at 1200, Exen 4/5 at 1150, and Exen 5/5 at 1300.
Note that changing from Lux to Agi Thokcha improves the Exenterator DPS, while it worsens the Evisceration DPS. Excluding any SA/TA, Cor+Lux w/Evis is 143 DPS against a DC target, the same as Cor+Thokcha w/Exen. Changing it so that no gear swaps are used at all, they drop to 138 DPS.
Keeping the three WS gear swaps, Cor+Lux w/Exen is 142 DPS (compared to 143 with Thokcha) at the expense of the Thokcha's 28 evasion. If you don't want another dagger, Cor+Lux provides roughly the same performance using either Evis or Exen; Exen just provides you with the extra evasion if you were lacking. So you'd have (relatively) low evasion with Evis, moderate with Exen (or Cor+Thokcha using Evis, if you wanted to drop Exenterator merits, though DPS would go down a bit), or high if you swapped Lux to Thokcha and used Exen. All configs would give fairly similar damage performance.
If I change the Evis set to your gear and Cor+Lux, and use no gear swaps at all, I get a 78 DPS, 55%-58% of my set. Or, considered another way, it would take you almost twice as long to kill the mob, without any significant defensive benefits since you should have effectively the same or better evasion using the Exen/Thokcha build.
My gear set would also do better on SA/TA (again, assuming no gear swaps, though I'd certainly recommend using AF3+2 hands), given that it has higher dex, higher agi, and additional crit damage, though I didn't include SA/TA damage in my calculations above.