I imagine I'm about to step on a few people toes here, but here's my opinion. This is basically how the most successful turbo'd rice burners over here in OZ get so much HP. WRX's, Lancers and skylines this is pretty much the common approach to getting big HP.
If I could offer one word of advice I would say these three words... INTERCOOLER INTERCOOLER INTERCOOLER, and not some small 40mm thick piece of ****. Have a look at how the skylines GTR's and GTS-T's, Mitsubishi EVO's and WRX's do their front mount IC's. Don't worry about piping lengths for the intercooler connections. These DON'T cause excessive lag provided you've matched your turbo's properly. Look at Anthony's Twin 1uzfe setup. Its close on 2 meters of piping from turbo to throttle body and it makes boost that quickly that the car can't get traction at all, in any gear for that matter.
Intercooler water spray activated via the engine management or a pressure switch is HIGHLY recommended. Water evaporating from an aluminium intercooler core when air is flowing thru it can bring the skin temp of the core to near 0 degrees, dramatically improving intercooler efficency. If your going realy extreme, use a CO2 gas spray system to literally freeze your intercooler and get your air temp close to 0 degrees.
Only after getting your intercooler to peak efficency should you look at water & methanol injection. Using water injection to raise the octane rating rather than using it to cool the air charge and suck up the heat.
Cut up your front bar like crazy, and fit the biggest core you can get. Consider buying a GTR or Supra front bar and adapting it for the job as part of the make over. These have big gaps for airflow to the intercooler. GTR's fit MASSIVE intercoolers in their front bars. Look at the GTR-700 at
http://www.exvitermini.com . 1300+ HP from an inline 6, and top quality intercooling the main reason its possible to run extreme boost.
Don't get some sort of core based on a truck intercooler either. They are inefficent and are not designed with perforance in mind. Anthony's core is 4" thick and the temp drop on it from compressor output to throttle body is amazing. Hot on one pipe and cold on the other. A noticable 30 degree+ drop. The more efficent your charge cooler the more boost you can run, the more ignition advance you can run and the more RELIABLE HP you will get.
Turbo's... well that depends on entirely how much HP you want to run. As for LAG well that will only be an issue if you don't have the right turbo for the job (Unless you've gone completely over the top and are pushing for more than 1000+ HP.
For something streetable, a pair of GT28RS 350HP turbo's would be ideal for 600HP at around 12-14lb boost. For more HP, upgrage to the GT28 with the 400 or 450hp trim for about 700-800 HP and 0 LAG. I think the GT30's 500 for is getting a little too close to the race end of the scale. In a twin system, a pair of GT30's would come on boost around 4000-4500 RPM, but would come on so hard that you would barely be able to hold on to the car. Fact is, if the road is wet you won't have a hope in hell of driving the car.
Engine management and injectors need to be matched to the job. Definatly an aftermarket ECU, autronic or wolf seems popular. High energy coils are a must, but consider upgrading to MSD digital 7 with 4 coils and a waste spark system if your shooting for more than 600 HP. You should also convert one of your distributors to a more accurate triggering system and run full sequential injection and larger injectors without suffering poor low end performance.
Thats about all I can think of for starters. One last thing... once a street car has around 500hp there is no way that you will want take your mother for a drive in it. Based on the lap Anthony took me for in his twin turbo commodore, that would give most people a heart attack.
Cheers,
Ian.