Hi bud. Yes, I think the Diana was only ever made to produce between 6 and 7 ft/lbs, like the other 10m match SSPs.
I read somewhere that around 600fps with a match pellet was found to be the optimum velocity for ultimate accuracy at 10m.
I'd be interested to know what sort of tests were run to arrive at this figure and if there's a similar 'optimum velocity' for guns running at higher muzzle energy.