We have been putting on the 165/60/15 and 175/55/15 Vredestein Quatrac tires. Customers are extremely happy with them and they test out well for wet and dry traction and stopping distance. They may not last as long as other options. We don't know yet. Typically, you will trade traction for longevity or vise-versa. But I am happy to trade some longevity for additional traction. Especially in wet conditions.
Most are getting the 175/55/15. Same diameter and about 3/8" wider tire.
For the rear, I believe the Kumho is about the only aftermarket tire you can get in the OEM 225/50/15 size anymore. But frankly, the Kumho is not a great tire. Wet traction is fair at best and even dry traction suffers as they near the end of life.
The General Altimax 43 & 45 test out better than the Kumho in both wet and dry conditions (as do other options mentioned). It is a 215/60/15 which is a bit taller than OEM (giving you a more accurate speedometer) and about 3/8" narrower than OEM. Unfortunately, the Veredestein comes only in a 205/60/15 which is a bit shorter than the Altimax, but still taller than OEM. It is an additional 3/8" narrower than the Altimax, (3/4" total narrower then OEM). However, another factor to consider is that at freeway speeds, the OEM Kenda is putting down only about 3"-3.5" of rubber on the road. This is why they wear out in the center. Reducing air pressure does little to correct this as the tread plies are simply too weak to maintain a flat surface against centrifugal force.
Even with the narrower Vredestein, you'll be getting about 8" of tire on the road surface at all times. This is what the Vredestein looks like.
Typically, a directional tire is better in wet conditions than a non-directional tire. With the tendency of the Spyder to hydroplane, especially in the rear, I like a water shedding tire.
There are some 225/60/15 tires that would probably work well. But also probably too tall as they would be about 1.7" taller than the OEM. Or about 3/4" closer to hard parts than OEM.
There are better tires out there than any so far mentioned. But none of them come in sizes that will fit our Spyder