You do rialise you will double your fuel usage! at lower revs, It should certainly go like a rocket at the lower end, but will make no difference at higher speed as the second choke would normally be in use by then anyway. I'm looking forward to seeing what you do though John, what is it you always say, Photo's mate :-)
Hmmm, I will be interested to see it Paul but to be honest it's not something I am interested in, I think these cars that people lower look strange and in a lot of cases I wonder how they get the thing off the drive without tearing the silencer off
My Corolla is 1998, and I'm sure just like your Previa, still has many years left in it.
You should only change if you need to change. If the car starts, stops, drives well and does everything you need it to, then why change it?