the BT 015, is the predecessor to the BT 016, which has been out for a few years now...
I'm guessing your 15's are at least 4+ years old.
Check the date stamp.
I wouldn't be pushing hard at all, just take it easy until you can afford some new tires.
You can always try and get a set of scrubs from your local track/race tire supplier.
PS Motorcycling isn't cheap.
2006 GSX-R 750 - Street
2006 GSX-R 600 - Track
2005 CBR600RR - Ex-Track