Well that sucks. That's the only way to alter the speedo on the cluster. An Insight CTS can account for the tire size but only on the Insight monitor, not the cluster
Negative. LMMs were the ones throwing a fit over the 4094 based turbo. Overboost if I recall correctly
LMLs don't have all the tables mapped out to get them to act right.
EDIT Some LMMs Underboost code P0299