Go and get yourself an aftermarket mechanical boost controller (bleed valve with a spring and ball bearing - holds the boost without bleed till the preset value of boost is reached). Good quality comes pretty cheap - Turboxs from Australia are nicely made (can't remember the UK distributor.)
Also from States - Joe P manual boost controller or Halmann MBC (www.buschurracing.com) - both for under 100 bucks.
Other option is a fancy HKS, Apex'i, Greddy, Blitz electronic boost controller. About 5 to 10 times the price with more things to go wrong - but you can map the boost curve with some of them - even in each gear with the latest Blitz one.
I have had a little experience with celica's but found the more you wound up the boost the slower it went because the ecu and knock sensor kept adjusting the timming back. ( went good on AVgas though.)