No experience with a Grob 103 but I tow with a short wing Piper 160hp. I could not tow a maxed 103 (1400 lbs?) due to a 1200 lb limit on the Piper’s tow hitch.
Last Summer, density altitude 3 to 4000’ , towing a maxed out 2-33, 220lb tow pilot and 30 gallons of fuel yielded about 300/350 fpm. Not stellar but completely safe as we departed from a paved surface.
Our tow plane is a PA22-108 with no interior and an stc’d O-320-B2A.. Weighs 1010 lbs empty. It is an extremely cost effective tug. We want to purchase a PW6 later this year and our intention is to continue to use the PA22 as our tug. The limit on the tow hitch and gross weight on the PW6 are matched. I suspect the PW6 will tow more easily than the 2-33 due to lower drag.