Just realized that even though I started the topic I never chimed in on the discussion, so I'll take a minute to add my two rupees
I'm kinda in the middle on this one... I upgrade some of my guns to varrying degrees, others I leave stock. If I plan to use the gun as a primary skirmish weapon (and I do skirmish with all my guns) I feel that it's mandatory to have a barrel tightness of at least 6.04mm, so all my "primary weapon" guns get tightbore inner barrels although I tend to leave sidearm barrels stock. Infact, most sidearms for me stay bone stock until something breaks unless there's some special custom project that I can afford and I'd like to do with that particular style of gun.
Primary skirmishing guns also tend to get upgrades for power as well (boosted hammer springs, high flow valves, etc.) and any upgrade parts needed to retain good durability at higher power levels.
There is an exception to this though, that being pretty much anything made by Western Arms. If it's a Western Arms gun I leave it stock even if I use it as a primary weapon, and my new 6" SVI is a good example of this. The reason I leave all WA guns stock is because I've found that unlike Marui and KSC guns, WA guns tend to become very unreliable when pushed past their factory specifications. Which basicly means upgrade nothing (expect for maybe the inner barrel, if you really want) and stick with 134a/duster gas and it should last a long time. But the moment you start messing with metal slides and enhanced springs expect to dump a good $300 to $800 into the gun before it will function reliably when pushed past the factory spec, and thats just more money than I care to spend. Besides, Western Arms pistols already have a 6.03mm inner barrel and I've always been happy with the stock preformance so WA guns in my arsenal always stay stock.
But Marui and most KSC guns... well, these guns generally preform very well and stay quite reliable even when pushed far beyond the factory spec so if the parts are available I have no problem upgrading them out the yin yang.
So yeah, as it stands I currently have 4 pistols (3 actually, 1 of them is on the way from Hong Kong). 2 of them are stock, 2 have various upgrades. To better explain what I'm talkin about, here's a quick list of how things look in my arsenal..
Western Arms 6.0" SV Infinity Classic (Black)
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Stock, although I will be upgrading the mag lips to Proud lips when I can afford them.
Tokyo Marui Desert Eagle .50AE Hardkick
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Nineball 6.03mm tightbore inner barrel, King Arms 170% Hammer Spring, Freedom Art Red Fiber Optic Sights
Tokyo Marui Biohazard Samurai Edge M9
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Stock, although I may be upgrading the hammer spring soon since the duster gas I'm using with it isn't providing enough power to make the slide lock back when the mag is empty most of the time.
Tanaka 8" M500 Revolver (Midnight Blue)
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Not here yet, but when it arrives it will get a DB Custom 6.01mm tightbore, real steel scope mount, some kind of handgun scope, AJAX black pearl grips, and be tuned to somewhere in the 550 fps range with green gas and 0.2g BBs.
So yeah, some stay stock unless there is a part I really need for reliability or proper function and others get upgraded quite a bit depending on what I want to use them for. Ultimately though for me it just comes to available cash and whether or not the pistol will retain it's reliabilty with upgrade parts installed.