did you simply swap out the slp in your sig to this one, and it started doing this ?
Did you buy it new or used ? If used it won't stall at the same rpm as on the previous owner's car if you aren't making the same kind of power. This would affect the street manners of your converter. If it's new and you're sure it doesn't stall at the advertised RPM, then Yank should stand behind their product and make it right for you (assuming you specified everything correctly).
Being at 2000 rpm at 40 mph is most likely due to your tune. I'd be very surprised if your current tune commanded 4th gear lockup as low as 40mph. You're probably still in 3rd gear.