you're talking about a suburban.... this is a passenger car we're talking about..
the 3% +/- has always been talked about.. even on here:
http://auto.howstuffworks.com/cg-tire-buying-tips3.htm
just changing to 20's doesn't really mean anything... it's about your overall tire circumference... meaning... tire size comes to play as well.
btw.. since the stock wheels are 215/45/17.. your 225/35/19 is about 2.4% slower... so that's fine..