This is incorrect. While it is true that a Phantom is an excellent gun out of the box, there are quite a few ways to increase both gas efficiency from 12 grams and accuracy. It's important to note that performing an individual mod to increase efficiency (like just replacing a spring, or just using a lightened hammer) will only result in marginal performance gains. To truly maximize gas efficiency (and see a noticeable difference in your shot count), you'll need to perform a few mods.
First, replace the stock hardline with larger SS-braided hose and a SS quick disconnect. While the hardline on a VSC is streamlined, it's a PITA to take off and does not really allow the gas to expand from the 12-gram to the valve. Replacing the hardline makes the gun easier to work on, and allow the gas to expand slightly inside the hose. When CO2 has a chance to expand a little you're not firing off pure liquid, which can give you a few more shots per 12 gram. Adding a small gas-through foregrip (that acts as a mini expansion chamber where the bucket changer screws in) is another option to help maximize shots and add consistency.
As mentioned, upgrading internals is part of the equation. New springs, and a lightened hammer can help. Some suggest cutting the top off the TPC in order to get it back to it's original configuration (Late 80's/Early 90's style velocity adjuster), but I can't say how much this affects efficiency. No matter what anyone tells you, polishing the internals will NOT increase effeciency.
Another way to increase efficiency (and accuracy) is by adding a bore-control barrel. Flasc, Lapco and other brands offer sized barrels that will eliminate blow-by. Blow-by is what happens when CO2 blows by a pellet that's smaller than the inner diameter of the barrel. Basically, you need to increase your velocity to compensate for the blow by which can drastically reduce your gas efficiency.
The single best way to maximize you shot count is to add a stabilizer or regulator. Palmer's stabs are the industry standard, but also come with a hefty price tag. Some say that the cost doesn't justify the gains, but if you truly want the best performance, then expect to pay for it.
Below are a few of my Phantoms. Each is equipped with an AGD Micro CA II or ACI Power Stock lever changer (all have check valves), a PPS Phantom Stabilizer, a sized barrel (either Lapco or Flasc), red main spring, SS hose and SS quick disconnect. The Phantom on top is shown disassembled and missing its bottom line adapter. The top two Phantoms have a 20 round feed tube and old-style pumps (about an inch longer than the current CCI pump). The bottom Phantom has an RTR frame and pump handle, and 15 round feed tube (the tape on the Micro CA lever is just a repair ticket). I have tried using an RTR valve body in the past, but had quite a few problems with it. I also hate one-piece feeds as they are far more difficult to clean than a two piece. Attachment 37473
Any of these guns will easily outperform a factory Phantom in both efficiency and accuracy. As configured, I get about 38/40 usable shots off of one 12 gram (using Leyland 12 grams, and either Nelson, Valken or RPS) in around 70°/75° temps @ approximately 287/292 FPS. I'd say that almost doubling the usable shot count of a 12 gram is rather significant. The key word here is "usable", meaning a shot that is able to eliminate (break on) an opponent at a "normal" range.
A factory Phantom is very accurate, but I've found that the bore control barrels do make a difference. They perform far better than any inserts, which still allow blow-by. The bottom line on accuracy is paint quality. A million performance mods won't help you if you have dimpled or out-of-round paint. When you do get good paint, then a nice barrel can really help. Since getting good paint is always a coin toss I don't get too hung up on the whole accuracy thing.
I guess you could say the above guns are my "Dream Phantoms" as they have all the performance mods to fit my style of play. The higher capacity feed, and high efficiency keep me in a duel longer before I need to replace gas and paint. I can change 12 grams in seconds, and I don't need to worry about small paint with a bore-control barrel. Any cosmetic mods like grips, or a custom ano can be cool, but a secondary consideration to performance.
I hope this info helps,