Do you mean if you got the "current line speed" value on the PN portal reduced to say "43.44" would upload go any higher?
No, it would have no effect.
Or do you mean if you had your actual sync speed reduced, for example by changing to a 40/10 package?
That is unlikely to have an effect on the upload speeds, as you are already close to 40Mbps.
However, BaldEagle did some experiemnts with different package settings a good while back, when he was having problems with his line. He *did* see some trade-off between the upstream and downstream speeds. That may have been because of the nature of his fault - ordinarily, there shouldn't be any trade-off visible.