Don't know about buying at Premium too much. I do know that when I sell on Premium, the whois changes to the buyers info within a day or two (never checked the actual time, but I did see the change within 2 days). I receive the money in my account on Day 5 (not counting the actual day it sold).
Got a question kinda related to this topic so I'll go ahead and post it here:
For GD Auctions BIN sales, when does one get paid? The buyer already paid GD, the 5 day period of verifying funds has ended, the domain has been pushed to the buyer (the push was done at another registrar, not GD). Yet, it's been like two weeks or something and GD hasn't paid out yet.