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On Youtube and google I didn't find out if anybody ever did a speedrun with SCS installed (and the improved final battles as well as precast spells and better AI/creatures).
I also don't know if an SCS speedrun would be much more interesting than a normal speedrun, since you'd have to cheese around many components that make SCS what it is and still wouldn't come remotely close to the times achieved in normal speedruns, since the Sarevok fight is quite a pain time wise.
On easy difficulty you'd at least have +6 luck which makes fireball/wands/necklace quite deadly.
As a character I tested barbarian which is doing quite well with longbow and his inherent movespeed.
You can easily threeshot the enemy with the deadshot on easy difficulty and even the comp bow +1 you can buy in Beregost is quite deadly.