Keith,
May I make an observation? As the sprocket rim is supported by 6 'Y' shaped legs, when the dish is pressed into it, it may pull the outer rim inwards in 6 places. Having dished the trial sprocket - check that it is still truly round - you may need to do the forming of the dish prior to cutting the sprocket teeth.
Also, any heat treatment of the teeth may result in more distortion - in practice, teeth left in the soft state are almost as durable as hardened teeth - so the extra work and problems may mean the soft option is better ( especially as high milages are less likely these days).
Regards,
Eddie.