I had Sole Performance in So Cal put new Sidi soles on a pair of my Tech 8's. The stock soles seemed to wear out quickly, and my kick starting style always broke the sole where the kick starter rested. The Sidi soles are a heavier (denser) rubber. Consequently they have not shown much wear at all, but they are quite a bit heavier than the stock Tech 8 soles. I think the price for resoling was $70 not including shipping costs. They don't change the comfort of the Tech 8 boot other than make them a heavier for walking, but then if you're riding...
I recommend the change, and Sole Performance did a very professional job.