5 might be too long

I think youre right. Otherwise everyone would wait 5 mins with the "last" bid and it would grow indefinitely. But 1 min might help. Though i dont know if its enough time to get around the trouble of not being allowed to post?
Hmm... There should be a guide at the OP to disable the warning where if someone posted first. 1 minute should be enough for those who has not done so anyway as the auction is already extended by 1 minutes.
Yeah, the secret ballot style auction might work too as I said above, but I sell prefer the auction extension style.More transparent.
If you don't want a sealed bid auction, how about a Dutch auction? Start high, and bids are fulfilled until the last item is sold. I reckon a Dutch auction would do well since you already have an indication of how well the ASICs will sell, and people are very keen not to miss out.