I'm not able to recommend on specific barrels for the Tipx. I use the standard barrel in my Tipxs, but if you were to change barrel you need a Tippmann threaded barrel, but without too much to the body (Any tapering etc will make the barrel fatter and it won't fit in the pistol frame.
I find additional accuracy in pistols due to the natural improved accuracy of pointing.
As a vest etc I recommend MOLLE, a MOLLE system of vest and pouches works out more expensive then a dedicated 'sewn' vest, but it's more versatile, especially for players like me who don't play in one way.
I could wear my vest to carry paintball pods, full size magazines, pistol magazines, pistol holsters*, grenades, general
* With a holster you can opt to not have one at all (going with the pistol in your hand all the time, a belt or vest mounted holster or a a drop leg holster.
I particularly hate drop leg holsters as they are down just out of reach, can drop your pistol when you kneel down, but the worst part for me is a weight flapping on my thigh.
I have an old DPM 'sewn' assault vest, a selection of MOLLE vests, and a yoked MOLLE belt kit.
There are plate carriers as an option but I find them too bulky. (They are more designed around covering your torso as body armour, acting as a platform is secondary)
My DPM vest was from an Army surplus shop, MOLLE vests were imported from the US (mostly SpecOps, OpsGear and Valken V-TAC), some other kit from Hong Kong etc. Things are much more available in the UK now.
For Tipx magazines I use an EBAirsoft magazine pouch, in which I mix magazines, 8 shot tubes and 12 grams, and I also use an Army surplus notebook case adapted for MOLLE with speedclips.
Some of my kit, see down the left for a vest and MOLLE belt.
(Though 99% of this is in MTP/Multicam/Dyecam/Vcam etc I don't always stick to matching kit, hence black & digtal DPM pouches
Pistol magazine pouch, this can be used with the flap cover on or off. One of the pictures shows how well the magazines stay even when shook upside down
Two holster I use:
