I've used both 110 grain and 130 grain SP's in mine. They both shoot really well. I've also used some 100 grain Hornandy "half jackets" that are a lot of fun.
The ONLY bullet that I have run accross that didn't perform well is a 180 grain cast lead bullet. I just can not get it to group, no matter what I try. Seems that the Encore barrel preffers the lighter lead bullets, but seems to shoot almost any jacketed bullet well.
I see no reason why you shouldn't have excellent results with the 170 grain Flat Points, but keep in mind that you might want to keep the velocities down. I think that most of the flat nosed bullets are designed to expand at lower velocites. If you push them too fast, they might disintegrate when they hit a target. Other than that, I would use them if I had them!