Don't really have an answer as I spend 90% of my time in the no-turning, lead-sled SU33. Is this in the context of a 'guns only' fight? Are you heavy at the time (gas, weapons)? My experience in the turn fight arena is that the -15 is the hardest to beat. It, as modeled, seems to have more available power at a given speed/config, which allows me to make minor errors yet stay in the 'saddle'.
Now, if the SU has R-73's it's diff since it can employ them earlier via the helmet-mounted sight. Two-circle fight would seem to make things worse (for the -15) since you'll have more separation when the SU has a shot, possibly giving the R73 a better chance.
However, according to this:
http://www.simhq.com/_air/air_097d.html
..you're not going to out-turn a -27. Ever. Assuming you're flying against your clone. Not sure how the author came up with the data however - and he seems not to delve into the slow-turn side of the envelope. You have me curious now too :)
Check-Six,
S77th-Beast