Speed difference with different weight pellets.

I have a HW100 that was chronographed at 1000fps with H&N FTT's that weigh 11.42 grains the accuracy is a shotgun pattern!! I shoot JSB Exact's that weigh 13.73 grains and it is a shooter with them. My question is how much speed difference is between the 2? The reason I ask is I was going to send it to someone to tune it down to about 850fps with the JSB's, but if the weight of the pellet already has it down close to 850fps I would save some money.
 
The only real way to know the speed difference for your gun/pellets is to find someone with a chrony.

Pellet weight (and shape) make a difference on impact point downrange. At the end of the barrel, yes weight matters, but so does head size, skirt size, pellet density, and friction between the pellet and barrel. My POI and group size changes even if I take the same JSB's and lube them prior to shooting.