I only worked for goodyear for about 2 years, so take this with a grain of salt. ADDITIONAL siping is only for poorly designed cheap tires, or those that you are using for a different purpose than originally designed. i.e. siping the middle blocks of mt'r's to get more block flex. However, you take away some of what you bought the mt'r for, but you have to decide if the trade off is good for you.
Tread compound is also really important, as that is what gives a good deal of the traction in lower temps. That is why blizzaks are so successful. They stay soft to lower temps.
The T/A is a GOOD all-round tire, and is hard to beat. However, find the lower mileage rated one if you can, as it has the softer compound. I would never sipe a tire, but that's me. I figure that those companies have huge engineering budgets for a reason!
j