Simple answer, for instant max power gain you want the Wingy Injectors, end of!
BUT, Without the simple mods it will smoke like crazy! The only thing you should be thinking of if you've got a remap on a standard car is refining that remap with the simpler bits first to make the most ou of what you've got.i.e. Mafam (£50ish), Decat (£30-40ish), Decent air filter(Vary), GBE valve(£40 ish) + Gauge(vary)(really important to know what boost your running), EGR blanking plates (£14ish)+ cleaning out the inlet pipe + manifold. Doing all these will transform the way your car drives and then set you up for any future power mods.
From there you can make the most out of a Exhaust system(£340ish) + High flow frontpipe(£150ish), Wingy Injectors(£245ish), FMIC(vary), Pocketbikeuk Hybrid Turbo(£400-500ish), Performance clutch(£350ish).
You can go even further with other things like Donnydiesel400(Mark) has on here. He's for sure got the most mods on an l series on this forum so it would be worth searching through his threads for extra info.
Hope this helps.
Cheers, Phil.