Most of the ATX PSU uses 2SC3150 or 2SC3866 or 2SC5027 transistors for +5VSB Primary Section . Look at those three transistors data sheet - they all have Vce is 800V . But if you work in ATX PSU then you will see that Collector is Actually crossing 1000V, may be it is 1500V to 2000V !!! I think this volt is generated by the Transformer not by the Transistor . Here you can use any transistor which Vce is greater than 320V and which can deliver more than 2A current for +5VSB section . I think it is such kind of design .
Actually - This PSU had 2SC3150 transistor which has burnt . I have Replaced it with MJE13005 . I have repaired many +5VSB unit with this MJE13005 , actually where was 2SC3150 or 2SC3866 or 2SC5027 transistor . And this MJE13005 collector also cross more than 1000V !!! So, I think this volt is generated by the transformer . You can see it easily in your mobile charger . 13001 or 13002 or 13003 collector is crossing more than 1000V !!! I have no choice of using 13005 because in my market - it is impossible to find 2SC3150 to 5027 . With 13005 +5VSB unit works absolutely Fine . And I have Repaired many Power Supply with this 13005 .
You said Base is 0.4V and it is normal to turn off the Transistor . But in some good Power supply 0.2V to 0.4V is enough to turn on the +5VSB SMPS unit . And some PSU I have seen base is in -1V to -2V . In both kind or PSU 13005 works perfectly . I have increased the Base voltage to 0.7V . The SMPS was turned on and I was getting more than 1000V but after 5 minutes 13005 and C945 both burnt due to excessive heat !! That means a good PSU need 0.2V to 0.4V to turn on .