The drive train you have wasnt used in the Bonneville till 2004 so I dont know of anyone who has high mileage yet. Being it'* a GXP which have their own little set of problems, and the fact that the 4T80E and The LD8 are major investments to replace if needed. I think it'* a wise idea to buy the Extended warranty. Good choice
The drive train is used in Caddy'* and is usually reliable beyond 100k but I still feel alot more comfortable having the extended warranty. My Dealer has been very gracious towards My mods as well warranty wise. At least till I break something expensive.