Nice work, especially in the merge area of the crossover.
Have you ever found any data that supports the idea that stepped, equal length headers are a benefit in a turbo application? With the backpressure I have thought there would be no or very little benefit.
In the studying I did when I was first putting together my engine configuration, I spent a lot of time looking at the high end turbo motorsports classes. Especially F1. Studying the F1 turbo era, I found that tuned length headers were very popular. In fact, I noticed that the era started out with more log style manifolds and evolved into tuned length. Now, these people have R&D resources at their disposal that I can't even begin to imagine. I would not dare to second guess their decisions to use tuned length headers. If they found there was power in that decision, then I was on the band wagon too. Another well known tuner, Steve Kinsler of Kinsler Fuel Injection has documented that a turbocharged engine reacts to cam and manifold tuning much the same way a naturally aspirated engine does. His quote on cam selection for a turbocharged engine is, "Don't choke that engine". Again, who am I to second guess his successful experiences. A lot of times you don't need to go through the expense to do your own testing to find answers to some tuning questions. Just look at what other highly successful people have done. And people that have much more resources for testing than a simple hobbiest racer.
On the question of stepped primaries. Few people realize this, but the perfect primary tube for a 4 stroke engine is actually one that has a continual taper to it, right up to the collector. Of course, shaping a complete header system with tapered primaries is a huge undertaking and is very expensive. I have actually seen a tapered primary system. Pretty cool. Study 2 stroke exhaust systems and you will understand the importance of the tapered primary idea for a 4 stroke application.
Since tapered primaries is beyond the realm of most people to afford, let alone fabricate, the next best solution is to step the primary diameter at calculated intervals. My primaries should have had another step, but I really didn't want to end up with merging three 1 7/8" primaries. As I look back on it, I wish I had gone ahead and put that extra step in it.
The Y-pipe is something I'm very proud of. In fabricating it I found an angle that gave the merge a very interesting quality. If you take the Y-pipe by itself and take an air gun to blow air into one manifold side of the Y-pipe, the air blast will exit through the turbo flange opening. The interesting thing is, none of the air blast will exit the opposite manifold side opening. But rather, a suction would be present at that side. I was very pleased with that result.