Which power adder:

[KPWSerpiente]

Furthermore, fitting an TT engine with two different sized turbos is a common belief, but does not happen. All stock mkIV TT comes with a pair of CT12B turbos, both exactly the same, but the engine management system is what is responsible for putting it into a sequential mode. Thus, the 2nd turbo does not actually "come on" until after the 1st has been running, but it has already spooled up, so it feels like the car is pulling even harder. A very common modification is to put the car into "true twin" configuration; eliminating the sequential setup and allowing them both to spool at the same time. The same applies to the 2.5L 1JZ-GTE sporting a pair of CT12As and all turbo upgrades that keep twin turbos have them both staying the same size.

I'm sorry, but a true sequential setup features one smaller turbo and one larger one. That is really the only true benefit to a twin setup...it provides good low rpm response with a high top end. As you say a twin set up with the same size turbos is a compromise and won't provide both.

http://auto.howstuffworks.com/turbo6.htm

If you can't believe me, trust howstuffworks.com.

The reason for this is because in the example you give, one turbo is driven off the exhaust gases and boosting, while the second comes "on line" and spools up. It is assumed that the 1st is approaching its maximum boost as the 2nd comes on. This is a terrific thought, except that the exhaust gases are still passing through the 1st turbo when the 2nd is going through its power band. There is a hell of a lot more gas coming out of the car than before (big 2nd turbo sucks in lots more air than small 1st turbo), and thus the 1st turbo will end up getting cooked from overboosting.
The VR-4 has two extremely small turbos, but they spool so amazingly quick that it is almost as if the car is NA (ie: virtually no lag at all)

I'm not an expert on sequential setups but I sincerely doubt this is a difficult thing to work around. If the wastegate on the first turbo dumped into the turbine of the second, this would solve your problem. Or if that isn't going to produce enough exhaust gas, how about an external gate on each runner dumping into the second turbo? I think the reason you don't see true sequential set ups is because they are expensive and require a lot of room in the engine bay, and because most people will either get by with a well selected single turbo. They're simply not practical.

What I was saying about engine size is that to power two decent size turbos, you need a larger engine. 2.0 liters is not going to cut it.

I'm pretty sure every car you mentioned is 3.0 liters or larger but I don't know about those Jags, I'm just guessing.
Interestingly enough, I got to thinking and the GTR is an exception to the rule.

-T