With a price of 750 at Perry, and 672 at TR, you should go with Perry. You will need to add taxes also to you bill. At the end, if you have a saving less than 50$, it don't make sense to buy it at TR. I am buying a lot of parts from US for my car (not a popular car here), and it is all a time more expensive than expected.
Not easy to do long event. Typically, it is differents runs groups, and because of that, the best is to lap 25 minutes per hour, which is quite a lot anyway for all day long. Tracktime wise, I am not sure that Time attack is the best.
For your question in regards of gain versus lost time with heavier tire... are you looking to fight the track record??? In theory, sticker tires will reduce the speed in straight, but will be faster on curve... good or bad??? Don't focus too much on that. With 100HP, try to have fun tires, not too grippy,