It's not just the boost pressure that's the issue but the volume of air it can produce. The T2 at 18psi still can't produce the volume of air required by th engine which is exactly why it's not a good option. The only upside to it is that it doesn't require hacking the car about to get it in. From a performance point of view though you really do need the T3 on there. A better spec'd Hybrid T3 is considerably cheaper than the Hybrid T2.

My standard T3 with 100k miles on it survived 3 years at 18psi and the only reason it died was because a vlave failed and bits went out the exhaust and through the turbo's splines scrapping it.