Actually, with power increase especially with incredible power increase, the previous lower level of power is increased as well.
For instance, during Trunks/android sags beginning, goku's supersaiyen power level was around 2million, however, during buu saga, this same super saiyen 1 level would have increased 20 fold along with the rest of his power, that is how DBZ power scale works.
So he would be much faster even without instant transmission by the time its comes around to GT.
He uses instant transmission because it is his fastest mode of travel, True, but it also has become mouch faster than it was way back when he met trunks and made that statement.
This makes that statement irrelevant and pointless.
What you said is like saying that hulks is at his strongest when he is base level.
Which we all know is untrue because he gets stronger and stronger, just like goku, and goku gets faster and faster too.
So your statement is balony, dude.
Goku has out run close range explosions(and that was only in freeza saga), Remember he gets stronger and faster.
He is well above lightspeed by the time he reaches supersaiyen 4 level.
That is even without "IT".
Oh, And instant transission is just that, instant, which means he can go from earth to pluto is several x the speed is light.
Just imagine how fast his running speed is.
(somehwere like, he dodges so fast that it appears that we go right through him, VErtor, he doesn't even appear to be movin'. I thought you we're the fastest in the universe, Vertor."😉