What is the current "stamp tax" percentage on property sales?

Five percent.
Paid at time of transfer of title.

Is it still 5% across the board for residents and non?

Yes - 5% for residents and non residents alike.

Actually it is calculated as follows:
the purchase price less $10,000.00 USD or $20,000.00 BZD then
5% of the remaining purchase price. If you purchase two properties (two separate titles) for a single price then you can divide the purchase price accordingly and subtract the $10,000.00 USD from each.