Never had good luck with yellow top optimas. That's rediculous that they won't stand up to their sales-pitch "warranty" though.
On the other hand, I've had the same red top in my truck for almost 6 years (2 more months). Ran it flat at least 6 times and didn't drive the thing for almost two years at one point. I'll be replacing it with another red top when it finally dies.
You're right about the thirst... I've actually gained 100km/tank with the 26/manual swap on low boost, but the 25/auto was horrible on gas. I'm sure the Legnum has it beat there.