it's not 40bhp because the JDM supra always put out more than the 280bhp in the spec.
There are many differences between the Euro/US and JDM engines. Most of the changes don't increase power but make the engine much better to mod. Injectors are larger (not really needed because the 440cc JDM injectors are not running at 100% anyway), stronger steel turbos, larger bore exhaust, MAF rather than MAP sensor, difference ECU mappings (Euro puts out more power and does better mpg). Boost may be a tiny bit higher.