had no problems with those sorts of speeds when we were in Spain:
293 average...
and yes anything to avoid the airport is a good thing.
friends from ky did a top gear style test to eastern creek in Syd.
so yes they are 150km closer to syd. but other guy used cars to and from airport not PT, was pretty well line ball.
2 hours to airport, park, to terminal check in 1 hour early, 1.5 hour flight wait for luggage, pick up rental car drive to venue.
and the drive was significantly cheaper.
CBD would be a different matter, with airport express v driving in, prob flight would be a bit faster.
Add to that how embarrassing getting to and from Melb airport is, t5axi car or a bloody bus, i've been to 3rd world countries that had a rail link.
if you could catch a train from any train station in melb, into southern cross and then 3 hours later be in Syd central station..my god it would be wonderful.