I have tested the following on my system, Win 98se, Nero 5.5 (It may be elsewhere with different version of Nero or of Windows!)
By a system of elimination I have found that the key is at the following;
HKEY_USERS\DEFAULT\Software\Ahead\Nero - Burning Rom\IsoDoc\Speed CD R
What you could do is;
Open Nero and set the speed you want as default, on the burn tab.
(This bit is important!) Start a new compilation.
You can then exit Nero.
Navigate to the above key, (in regedit), highlight it.
From the "Registry" menu in regedit select "Export Registry file" and choose "Selected branch"
Save it to a place of your choosing.
Subsequently, right clicking on the saved file, and choosing "Merge" will put that value back in place.
I hope this helps, and I have understood you request correctly!!