I am not sure that is the case. It has a much improved Visual Confimation that so far as I know the bot writers have not cracked yet but what else is there that is in there to stop them?
Of course if you had installed the RAC mod in the first place then you never would have had those 1000's of spambot registrations.
Are you sure that the upgrade to phpBB3 is just as easy as installing the RAC mod.
A test - stop watches ready lads.
The truth is out there.
Unfortunately they will not let you anywhere near it!