It will be as big a jump or more. Bmw for some bizarre reason started to underrate their turbo engines and continue to do so when they never ever underated their non-turbo engines and probably overrated slightly.
So 415hp for the new m3 is likely more like 450. 300 pounds lighter is another 35 equivelent hp to the current e9x m3. So you are looking at a big performance jump of about 70 hp which is essentially the same jump as e4 to e9