"For the motor octane number when you reach 100% isooctane you cant of course mix more isooctane"
By definition the octane rating of isooctane is 100. Both 100 RON and 100 MON, with RON meaning Research Octane Number and MON - Motor Octane Number. These tests are done on 2 different types of Octane engine. The MON rating of a particular fuel is always lower than the RON rating of the same fuel. This is because the MON method is a harsher test on the fuel than the RON method. There are plenty of fuels that have an octane number greater than 100, both MON and RON. By "fuel" I'm not meaning necessarily commercially available fuels but any hydrocarbon that can be successfully run on the laboratory internal combustion engines.
"I’ve heard the ”Performance number” before but as often Ive also heard octane numbers above 100."
Octane numbers refer to fuels tested on normally aspirated laboratory engines (F1 or F2) while Performance Numbers refer to tests performed on a supercharged laboratory engine (F4 engine). So 3 different types of tests, 2 types of Octane number, 1 Performance Number. All 3 can be greater than 100. Performance Number - always, MON - always for Avgas, not for motor gasolines, RON - never for regular commercial fuels (Avgas is not tested on the RON engine but it would be higher than 100) but Toluene for example is over 120 if my memory is correct..