Thanks Smoothe for the consideration but you are more of a turbo guy then I....I very familiar with nitrous and superchargering but have a good knowledge about turbo charging these SC400, LS00's as well...
Why twins?...More money, more turbo piping, more can go wrong.... A single turbo would be a more sensible choice on these 400's... My good buddy just single turbo charged his SC400...It delivers 300whp on the stock fuel injectors and stock ECU... Get on Ebay and get a used turbo, install electric fans to frre up room (flex-a-lites) fab the turbo piping, install an intercooler, etc... If you know what you are doing you can get it done for around $1000-$1500.... Alot of custom fab work..
If you really want to learn about turbo charging your sc, check out lextreme.com and or Clublexus and search...
Just so know, there are no ready made turbo kits available for the SC4's...There are supercharger kits for around $5000...Eaton chargers are favorite and can give you 100 more hp or so....