Most of those cars mentioned with sequential setups cost more than the houses we live in and add a few zeros for the companies R&D budget. The only sequential turbocharged cars available (off the top of my head, correct me if i'm wrong) in the sub 100k catergory in austrailia are the subaru liberty B4 and the Mazda RX7, the later i have had alot of experience with through my workings with Mazda Motorsports race program from a couple of years ago.
To go a sequential setup it needs to be very accurately setup and tuned. Issues with turbulence and overspeeding the primary turbo (a common problem with both the RX7 and the B4) come into play and it's too hard (in my opinon) for we street tuners and backyard boys to play with.
As ddgonzal said it would be possible to set it up as a turbo per 2 cylinders and run it like that and then its just an issue of changing the back housings and wheels to reduce lag (on the T25) to make it more drivable.
Like L18 and Timo said 2xT25's is alot of air to push for only a 1.8.
I still think S15BB is the go, and that is my rant for friday