testnet, for the time being, has been > 32 bits. Checking if H <= target (instead of H == 0 and then G <= in the host code) would slow it down. I could add it, but it'd cause complexity that might slow it down for normal users if I do it wrong.
Well, but this check is performed only when possible valid block is found, right? So we are talking about 'slowing down' once per tens of seconds or even minutes for every worker (depends on target and machine speed).
I'm asking that because strict checking on workers are much 'cheaper' than lazy checking on workers and full check on central server. For saving server resources we need to distribute full target checking as much as possible...