What is the ideal play through path for the GOTY Edition of Borderlands?

I am back on a Borderlands kick and am at a loss for an "optimized" play though sequence of play though 1 & 2 of the core game and the 3 expansions (Zombie Island, General Knoxx and Claptrap's).

I know that play through 2.5 (meeting the evil in a can a second time) makes all the mobs your level for the rest of the core game. Does that also bump play though 2 mobs in the DLC to your level if you haven't completed them? How about DLC play though 1 mobs?

There seems to be way more content than leveling available and I rather avoid facerolling as much as possible.

Solution 1:

I played Claptrap's New Robot Revolution PT 2 last and wish I had played it first (after the main game playthrough of course). Completing the Robolution story mission provides you with a reward that can benefit the rest of your playthroughs (spoiler):

It provides you with a shopping spree which is a room packed full of level 69 chests. You can infinitely reload your game and return to the room for new loot.

So from start to finish I would suggest the following:

1. Main Game PT 1
2. Claptrap's New Robot Revolution PT 1
3. Main Game PT 2
4. Claptrap's New Robot Revolution PT 2
5. The Secret Armory of General Knoxx PT 2 [Spoiler] Decent reward while the detonation counts down.
6. Zombie Island of Dr. Ned PT 2
7. Mad Moxxi's Underdome Riot (However access the bank here as soon as you gain access. Bank allows you to deposit, and later withdraw, up to 42 items from your backpack.)

Solution 2:

From http://gbxforums.gearboxsoftware.com/showthread.php?t=112350

|Main Game PT 1| --> |Main Game PT 2| --> |Moxxie PT 2| --> |Knoxx PT 2| --> |ZI PT 2| --> |Robolution PT 2| --> |scaled PT 2.5 free-play everywhere!|

The link gives you much more information.

Solution 3:

For story continuity, prevention of spoilers, and order of release:
vanilla/main content → The Zombie Island of Dr. Ned (DLC1) → Mad Moxxi's Underdome Riot (DLC2) → The Secret Armory of General Knoxx (DLC3) → Claptrap's New Robot Revolution (DLC4, Robolution).

I know that play through 2.5 (meeting the evil in a can a second time) makes all the mobs your level for the rest of the core game. Does that also bump play though 2 mobs in the DLC to your level if you haven't completed them? How about DLC play though 1 mobs?

"Meeting the evil in a can a second time" does not "also bump [playthrough] 2 mobs" and/nor "[playthrough] 1 mobs in the DLC to your level" whether or not that you have or "you haven't completed" DLC. For more information on dynamic scaling (triggers), see this thread.