The stock offset is +55, so a +35 rim will be 20mm less space between the hub of the rim and the rim's center. If there is a clearance problem you could always put on wheel spacers.
The CT's
entire wheel spec is:
Rims/wheels: 18"x7" on all 4 wheels, offset is 55mm, bolt pattern is 5 x 114.3, while the centre bore is 64.1 mm.
Obviously make sure the bolt pattern is the same (it's a common Japanese bolt pattern). Most stock wheels are "hubcentric" meaning the bore through the center of the wheel is what the wheel sits on, most aftermarket wheels are not hubcentric because hubs vary in size somewhat. Sometimes they put a little ring in the wheel hub to make it hubcentric and sometimes they don't and you just rely on the lug bolts to center & hold the wheel. I've had wheels that are not hubcentric (on the street) with no noticeable ill effects.
Here is the easiest to use calculator on changing tire sizes and what that does to your speedo:
Miata Tire Calculator. So you can compare the stock tire size to the ones you mentioned (remember you probably want the widest one you can get to protect the rim lip against rash).
I don't know how much you are doing this for aesthetics and how much you care about performance but if those rims are less than like $8k they are going to be forged or cast from a pretty heavy alloy. 20" rims and tires don't have like 5% more mass (all things being equal) than 18"s, they have like 15-20% more mass.
You will be adding not insignifigant unsprung weight (vs. sprung weight, as in overall weight of the car, which matters just "a little," unsprung matter a *lot*) to each hub. . .everything "good" suffers (acceleration, braking, cornering, MPG, etc.) From the
tire rack:
Reduced Unsprung Weight Compared to Steel Wheels
This is one of the most critical factors affecting a vehicle's road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight, alloy wheels provide more precise steering input and improved "turning in" characteristics.
This is why wheels, true performance ones, like HRE or BBS are extraordinarily expensive - it's because they tend to be very light weight. This is also why in all the sets of rims I've ever owned, I've only owned one set of BBS wheels. However I do weigh my wheel/tire combos to see what, if any, damage I may be doing to my pride.
This is also why you should never feel threatened in a stop light drag race against anything sporting "
donks."
Here are a bunch of great links on actual tests on heavier wheel/tires.
http://www.rx8club.com/showthread.php?t=51865
-ace