There's been much speculation as to the merits of aftermarket vs. OEM DV's, and the inclusion of the spacer.
I've had the old style rubber diaphragm OEM valve, the new style solid OEM DV valve, a Forge FSiT valve, and the spacer (which I still have sat in a box somewhere).
I've always been led to believe that the reason why the DV was relocated on the KO4 cars was in part down to heat management (there had been OEM DV's sticking on the K03 models where it was located on the turbo housing and subjected to high temps), hence the coldside relocation alleviated this issue. A benefit is it's easier to work on, but I've sometimes questioned whether having the DV so far from the turbo has any knock on effects as the pipework is long, has a right angle bend in its flowpath, is subject to high temperatures and can sag when hot (unless you get Forge one). I've also wondered if it contributes to the 'flutter' sound on the K04 cars, but best not to open that debate here.
I had a very early Forge DV, and it felt to me like the DV was reacting slower than the electronic OEM unit. Just a perception, but after stage 2+ I reverted to a revision D OEM valve and never had any issues. I'm not saying there is anything wrong with the Forge valve, it's a stunning piece of engineering, I think it may just have been a spring incompatibility issue with the state of tune that I was running,
I tried the Forge spacer as someone who owned an Ibiza Cupra bought the wrong one and gifted it to me dirt cheap. Yes it made an 'interesting' noise, but not particularly head turning. One thing I did note when I took it off was there was a film of oil misting inside the venting holes (no fault of the product, just a sign of the way the car was running), and this was despite running a catch can. I felt that it made the car feel a touch laggy, but we're talking an almost imperceptible difference.
I got the best overall sound with a Forge DV, ITG CAI, and the full TBE as everything was breathing properly. Currently running OEM engine cover and exhaust system on the new car and it's like the car has asthma!