It does make a very noticable difference. 3.08 to 3.45 is a pretty big jump. And with the new 276 pushing the powerband up slightly higher than the old 268 was it definately helps it get up and boogy quicker.
0-60 even driving normally is much much better. I wanted to rebuild this LSD centre with 3.7's later down the track but I'm thinking this is actually the sweet spot for me now. As I live 45km out of Adelaide via a 110km/h freeway the 3.45's seem to rev really nicely on the freeway. Not being a daily I might still change my mind down the track and want 3.7's, who knows the car isn't a daily so it really doesn't make much difference in the bigger picture.
I've ordered a speedo corrector from Jaycar, I've read a few mixed reviews on the current version so we'll see how we go. There aren't really any other alternatives within a similar price bracket, the Jaycar unit is $50ish and the others all seem to be at least $200+