We are still implementing on 10.1.600…We do have a one issue with our PDT results at the moment, with regards to the SQL query test. where the time is 4850ms when then pass value is 3500 I recall. I believe part of the reason is our SQL box is on a separate subnet. Still trying to get that worked out.
With 15 concurrent users I would have thought you could get away with SQL and App on the same box. Of course review the hardware sizing guide. and as @askulte mentioned the PDT is your friend, at least to ensure that your machine is configured correctly and you can baseline. The Tuning guide and Sizing guide are a pretty good places to start when you are looking at these things.