If it's not really about tuning or wiring. Then it belongs in here.
Lets say I have a perfectly tuned fuel/ ignition map for an engine running on 95 octane. How much would you guess I could advance the timing (overall) if I went to 100 octane? Anyone having practical experience with this?
Depends on the car, the compression ratio, boost etc.
On Volvo T5s that I have some experience with I can say that at peak torque (3500-4000) I can run 3-4 deg more timing before the knock control of the stock Motronic ECU starts pulling timing. The rest of the rpm range pretty much the same but most noticeable at 3500-4000.
If the car is not boosted it might not be knock limited anyways so you won't gain anything by using 100 octane instead of 95.