After having used G8s in a few RL games I can say that the question is only an issue for rules nerds!
In practice it's all about communication with your opponent and without having thought all of these semantics and whatnots through we ended up playing it as a pre-notch effect.
At the end of the day, the rule is that once you are notched you are locked in to the maneuver and the speed so the moving player has the final say on how fast they are going. It seemed counter-intuitive to say to them "ok, you notched that speed 3 maneuver, well tough bananas you are now going speed 2"
It always ended up being about talking it through with the opponent ("hey, see that star destroyer? hope you have some nav commands in that fat stack of dials or don't expect to move it again hahahahha") and we just settled on the interpretation above where you CAN increase your permanent speed to counter the G8 and the G8 has to be declared before you notch and it seemed to work fine and was fun. a few crucial moves were preceded by a "WAIT!" before a tool was notched followed by a "I think im going to G8. WAIT. No, I'm not. WAIT! no ok i'm not" lol. Its still quite powerful, I mean I used them plus tractors plus Konstantine to manhandle an enemy ISD to block an enemy demolisher and then slowed the demolisher after it bumped the ISD so it never got above an effective speed 2.