From what I've seen, gasoline eninges use compound seq turbos a bit differently. Diesels try to get the extra boost, gasoline want the extra flow by using a larger primary. What I should have said with the max flow deal is the larger turbo makes more than it normally would since you can use such a large turbine. That DSM guy said he was looking for the largest T4 turbine he could find.
That diagram is spot on, minus the check valve. I recall someone trying that on a home made setup, and it usually stuck closed. Since you're dealing with compressed air through the small turbo, you can basicly pass much more air through it when the large turbo is at 1 Bar of boost.
The key on a gasoline engine is to keep restriction down. Diesels are okay since they are used to high backpressure (drive pressure). THe thing about the WG sizing is that it should be sized for very low boost, since there won't me as high a pressure drop, and therefore it doesn't want to move as much air.