I was not aware until today that there was a service available from various tire retailers to sipe a non-siped tire. Apparently, this is advertised to extend the life of your tires, make them run cooler and increase grip. But if you already read my post on how snow tires work, then you would know that something seems fishy with these statements.
The tire I saw this done on was a Nitto NT420s SUV tire which is more summer oriented. These tires have long triangular tread blocks that already have two sipes in them, but the addition of many more sipes cut into them 90° to the direction of the tread really looks like it took out a lot of stability from each block. Also, the tips of these long triangles were now totally separated from the rest of the block and looked like they could now be easily torn off. If you compare this tire to a tire that’s siped from the factory, like a good snow tire, you will see that each block has a very balanced amount of siping that doesn’t leave small portions of the tread blocks unsupported like this. It’s quite obvious that this Nitto tire was never designed to be siped.
Siping makes the tread blocks squirm more, decreasing response and lowering traction on hard surfaces; only on soft surfaces like snow, dirt and mud do they increase traction. Sipes also don’t increase traction on a wet roads, but they do prevent hydroplaning by providing more areas for water evacuation.
Siping also doesn’t help cool your tires, in fact, the opposite happens due to the friction created between the sipes. I have even seen people cut sipes into racing slicks for short auto-crosses to help build heat; though the spacing was much wider on those.
Unless I was trying to make some budget snow tires, I would stay away from any aftermarket siping. I feel that the only result of this would be damaging and shortening the life of the tires.