The difference on paper is close to 100 horsepower but in practice it is closer to 200 horses as the new M3 is severely underrated. Horsepower is only one part of this story as the E30 M3 DTM car is much lighter and stripped out for the track. The weight difference is in the 1000 pound range based on a 3562 pound curb weight for the F80 M3.
Can a chassis design that is over three decades old now compete with a brand new and much more powerful M3? Yes. Not only does the 1992 E30 M3 DTM car compete with the 2015 F80 M3, it beats it around the track. Keep in mind that back in the 80's and early 90's DTM cars actually represented their production counterparts.
The E30 M3 does a 1:09.1 laptime around the Hockenheim to the F80 M3 1:13.1. A four second difference which is simply stunning. The new F80 M3 is a great car but it hardly makes the classic M's obsolete. Namely on the racetrack which the BMW Motorsport division used to design their M3's for.