That's version 2 mate, the guy doesn't know what he's talking about. This confusion often arises with the chassis code, as version 2 = GC8C. Not hard to see why they somethings think they have a V3. V3's are GC8D. (GC8A = "Version 0" that's why).
As for pro's and cons of the V3 or EJ20K intake setup and loom.
Pro's: No failure prone coil packs, yellow 440cc injectors, bigger dual entry intercooler, newer sensors, newer wiring? There's not allot to be honest, the main reason is to get the yellows in there while using a stock ECU. It's a null point if you go aftermarket management in future.
Cons: You gotta lean what is different, what's the same, and how to mix and match the two. While entirely possible and easy for someone who knows these engines in and out, it can be hard for someone unfamiliar, and allot of money can be lost in buying parts that don't work, or are said to be, for example, EJ20K parts, when they are infact what you already got - EJ20G parts.
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I should probably make this clear.
Gen 1 legacy = BC5 (sedan), or BF5 (wagon), and they have EJ20G's. Legacy gear has additional wiring to run the water pump for the water to air intercooler, and the ECU has pins for this too.
Version 1 and 2 impreza = GC8B and GC8C (GF for wagon). These also have EJ20G's, there are some differences, these actually have the second generation of EJ20G, but the intake manifold, throttle body, turbo intake etc. is the same. The main difference is they have an air to air intercooler, and so no wiring for an intercooler water pump. Other wise though, more or less the same as the legacy EJ20G's.
Version 3 and 4 impreza = GC8D and GC8E. This is the EJ20K generation, with the 3 plug loom and yellow top injectors, bigger dual entry intercooler, etc. etc.
BUT HERE'S WHERE IT GETS TRICKY. The EJ20G was also available for this generation. This, the 3rd version of the G was found in foresters, auto's, and wrx wagons for version 3 and 4.. Manual WRX sedans, and all STi's got the EJ20K. The 3rd rev of the 20G however is more or less exactly the same on the outside as the 20K, so new loom, intake, intercooler, coil pack setup etc. The difference lies in the ECU, injectors, and engine internals. The 20G's still ran grey top 380cc injectors, but the loom and stuff is all 3-plug like the STi's EJ20K. So it's a bit of a mix'n'match.
They can be useful, if you want to get a 3-plug loom going, you can use one of those ECU's, and not have to worry bout the yellow injectors and orange AFM, good for a "i've swapped loom, but haven't got round to the STI bits yet" sort of deal. I've known quite a few people who've done this to iron out things before commiting to the STi/EJ20K parts upgrade.
For more on what the chassis codes mean, check here
http://www.clubsub.org.nz/forum/index.php/topic,11849.0.html
And a useful, and mostly accurate decoder can be found here
http://www.scoobyprojects.co.uk/technical/appliedmodel.shtml