I can see some fine cracks in the giubo/flex disc on my LS1 Lumina's driveshaft.
Is this crack size serious enough to warrant a replacement?
Absolutely. Remember what I wrote about stress risers. It doesn't matter if it's metal or rubber, the stress riser will behave the same. This is the beginning stages of wearing out. It won't be long, especially after the vehicle is running again, for it to be completely toast.
On a separate note, since I have foreknowledge of what you are doing with the vehicle, it would make more sense to put a driveline in there which has u-joints instead of the guibo. If you are racing your vehicle, the guibo is going to be a weak link in your vehicle and will suffer catastrophic loss on a regular basis. Putting a high performance drive shaft in will serve you for many years to come. Up front cost will be something you'll have to deal with, but it shouldn't be over the top. Much better than losing a drive shaft on a hard launch.