Need for speed
I don't know anything about the Jett Fire Engines. I do know the sport .46 is a great engine, at a very good price (In comparison).
It has an idleable carb (that actually allows it to idle) and puts out great power.
This engine could be run in both Fast planes, and / or sport planes. They are beutifully built, and will last a long time if taken care of.
Its tough to beat the price, and it will get you to 150 mph.
170 mph is very tough to do in a quickee. That is very close to the fastest quickee I have ever seen with a Nelson .40. Also, you are talking about a wood/foam quickee, and not a composite.
Lastly, if you have alot of experience with these types of engines, the fire may be the way to go. If this will be your first "extreme speed" plane, I would suggest the Sport Jett. You will have a much easier time learning how to set the needle and choosing the proper props. At the same time, allowing you to throttle back if the speed is too great, or the plane is outta trim.
Good Luck.