Cracks on tires?
Not a real good sign:
The sidewall covering protects the cords from sunlight, moisture and air, which I understand may cause deterition, weakening of the carcass.
I am thinking these are old stock. Did you get these as thru a "Blow-out" sale or special?
I wouldn't guarantee they will cause you problems down the line, but if I had my choice...I wouldn't make the purchase (unless the price was damn good, I am a cheap bastard after all - it's human nature).
The point is, racing is dangerous enough, under the best circumstances...so adding something that causes you pause, think...might not be a good thing, if you can help it.
Slicks are going to work the sidewall, especially if you drop the pressure, a lot more than regular tires.
On the other hand, it may not matter, it might be a result of the construction of the silck, they may use a very thin coat on the sidewall, because unlike street tires, you don't expect them to be rubbing on curbs when parking.
If you are real concerned, see if the mfg. has a help line that you can call for info, advice. Might help make you feel better about them?