I understand what you're saying about the HKS kit. I've considered that one as well, and just figured they were larger than my needs. However, I have heard of people doing the GT2825 turbos (I believe that's the right one) on the rest of the HKS kit. Talked about virtually no lag at all, but was limited to ~650hp. It's been a while since I read about that setup, so my numbers could be off.
People usually have two meanings to the word "lag." So for the sake of clearing things up, lag is the time between when the throttle is opened and when the turbo reaches the steady-state RPM for the engine RPM currently at. For example, lets say you have a small turbo that is set to put out 15 psi of boost at 4500 RPM. You're driving down the road with the tach at 4500 RPM then step on the throttle. Lag = how much time it takes the turbo to reach 15 psi.
The other name that commonly gets lumped in to "lag" is called "boost threshold." This deals with what boost pressure can be achieved at a given RPM under steady-state conditions. Even with a small single, you're not going to make any boost at 1500 RPM no matter how long you wait there. The simplest way for most people to test this is to do a 3rd or 4th gear pull down the freeway, or run on a chassis dyno. It should be defined by a boost vs RPM curve, but some people define it as a single RPM point - the RPM where the turbo first starts making boost. Other people define that point where it reaches their set point for boost control (kind of bad definition because it will vary depending on what your boost is set at).
Lag has more to do with the rotational mass of the turbo, and friction of the bearings. Boost threshold is really more the size of the turbo. Ball bearings usually improve lag quite a bit, while they do little to effect boost threshold.
With that background, running twins usually doesn't improve boost threshold. Given a single, or a set of twins (assuming parallel) that are capable of similar power output, both setups will likely have a very similar boost threshold. However, where twins absolutely outperform singles is in lag. The rotational inertia increases by a factor of radius^2. There ends up being a significant improvement in reducing moment of inertia by running two small turbos over one larger one capable of the same flow.
But all turbos aren't created equal either. Two small journal bearing turbos (read: stock twins) are going to have more lag than a similarly sized bb single turbo. But two bb twins should have a distinct advantage over any single when it comes to lag.
All that being said, I don't know if the Blitz kit uses BB turbos or not. I see that they're using KKK26 housings (same as on the older 5-cylinder turbo models). I know mine weren't bb turbos on my 5kcstq, but the ones in the Blitz kit may be. One thing for sure....they are tough turbos. My first Audi didn't come with a BOV or bypass valve, and the original turbo was still running strong at 175k miles!