Be carefull with "Sympathetic resonance" , some marketing products are sold with " real Sympathetic resonance" and it's only convolution or reverb simulation.
Sympathetic resonance is more than a sustain effect :
it's rules between keys that are undampered. (Pedal up or down, the effect depend on the number of undampered keys and some rules)
FOr example, Ivory Claim to have sympathetic resonance but it's only a sustain effect when pedal is down. When keys are played pedal up, the behaviour is unreal.
Steinway D of Kontakt of a basic sympathetic resonance" and AKP have a real sympathetic resonance and enhanced sympathetic resonance in the next upcoming update.
Interesting!!! There is a script page for Kontakt 2 at: http://music.mezo.com/
...for various pianos which creates an impression of sympathetic resonance.
I thought of buying one of these, (as I have K2) but as you mentioned that AKP has a certain (better) type of sympathetic resonance and Ivory another I'm not sure what to do! Perhaps I should just go for AKP as it has the K2 engine already.
However, Sampletekk have some good deals right now, - Black Grand etc for which Kornel mezo has written a script.