What michelin Super Sports can i get for 18x8s without having speed issues, or the speedometer being off

A) Great tire. In fact I think the Pilot Super Sport is the best summer tire out there. I run the 205/45/17 on my F56 and the 255/35/18 on my E46
B) I'd say 205/40/18 but they don't make a Super Sport in that size. There is an all season version in the A/S Pilot Sport that comes in that size however, Tirerack data show it performs almost as well but I've never tried it. (I live in Vegas, what would I do with an all season tire?)
C) 215/40/18 is the smallest 18" Super Sport. It will work on an 18x8 rim ... just not sure how the added height/width with work with the rim on the car. I'd worry more about rubbing then the small amount the speedo would be off.

What size tire are you running now?? If your new tire diameter is <5% different, I wouldn't worry about the speedo error. Our cars generally read high on the speed anyway due to various reasons. I know my speedo is always 2 mph high vs any of the many radar speed signs around my area.

Right now i have a set of JCW 17s, with a 205/45/R17

I cant imagine the bigger wheel would do anything to the speedo

I have them as well, and they are beast, even in frigid weather, they have kept me safe and sound all winter (in Georgia)

How do those 4s sports hold up? I really do not want to spend almost 300 per tire.

So I kept with 17" x7.5" wheels and was able to use the 215/45/17 which I just checked on tirerack for $184. I don't know how, but I actually paid $159 a tire. I see now it was an exntremely good deal for $159. Performance wise, they are above and beyond anything else I have ridden on. They keep traction on both wet and dry pavement in just about all situtations except the most determined burnouts (read me accelerating too quickly). Otherwise, I created some pretty heavy lift-off oversteer in a downpour situation doing felon speeds around a turn once. (as you can see I was pushing the car far beyond its limits, and it still grabbed hard and corrected easily after the slide. In terms of wear, I can't really tell you. I had Michelin AS 3+ before, and they only lasted 25k miles despite lasting more than 45k on my SOs car. The difference I believe came down to the fact that I was constantly reaching above the limits on the AS 3+, and one thing that absolutely kills tires is burning them up. At this point, I have just over 7k miles on the PS 4s and honestly, they look like they will last at least 3 times this, having only lost a mm, the last time I checked. If I get over 20k from them, I will be extremely happy; there is a chance I will get slightly more.

TLDR: They are great and in my size, super sticky, and were cheaper than most alternatives.

EDIT: I see you have 18"x8"s, you should be able to get away 225/40/18 and tireracks cost for that is $198. Are you in the US?