The Vault Hunter's Relic only drops the chances of white item drops by 5% and then splits that 5% between the green and blue drops. so say if the odds of drops are:
No drop:40%, white:30%, green:15%, blue:10%, purple:5%, orange:Only on named & bosses (and often about 1% or less there) OR 3 borderlands symbols on a slot machine (about a 1 in 5,000 chance)
The Vault Hunters Relic would only make that:
No drop:40%, white:25%, green:18%, blue:12%, purple:5%, orange:Only on named & bosses (and often about 1% or less there) OR 3 borderlands symbols on a slot machine (about a 1 in 5,000 chance)
assuming those numbers were accurate (the orange is pretty close but the others I am mearly speculating) then even if it were modded to be a 30% better chance (thus eliminating white drops all together) it would still only be about: None:40%, White:0%, Green:38%, Blue:17%, Purple:5%, Orange:special.
No modding of %'s will make the Vault Hunters Relic really worth wearing once others become an option. Better to wear a relic that works to improve your strengths (such as an elemental boost on a siren or a brand-specific relic that helps boost your best weapon) or compensate for a weakness (+shield capacity or max health if you find yourself dying too easily).
The Wiki says there are 5 new weapons introduced in the DLC. There are four new Uniques (blue): The Cobra, The Pocket Rocket, The Boom Puppy, and The Kitten, as well as a new E-Tech shotgun called the Slow Hand. There are no new Legendaries sadly. All of the orange Tourgue weapons in the vending machines can be found as possible drops in the general campaign.
Best Answer
Yes, you can spend your golden keys and cash now, a level 50 gun is a level 50 gun. Playthrough 2.5 is only important insofar as it makes getting level 50 guns much easier.
Playthrough 2.5 is important for two things:
See How do subsequent playthroughs work? for info on quests (you're clear to take quests now, as long as they're level 50), but the main thing about playthrough 2.5 is that is scales enemies to level 50. Better stuff doesn't drop in playthrough 2.5 per se, it's just going to be (up to) level 50, so it's the best stuff. Technically you can go back in playthrough 2 and find non-level 50 stuff which will still drop lower 50 guns.