-My understanding is that Concept 2 designed the foot stretcher angle to be used with running shoes which have an elevated heel.
-However, running shoes are not the best for energy transfer because they employ a soft sole.
-I use weight lifting shoes, Adidas Powerlift 4, which have a very hard sole, but have a elevated heel and a low toe. I wear them really loose so my heel can pivot, seems to work well for me. They could use more ventilation though.
-I also tried Crossfit Nano 4s for quite awhile which are minimalist and flat and fairly firm soled. These were great for energy transfer because they didn't have much compressible material under the foot so the drive felt really connected.
-I found that having a heel helped me personally because I didn't have to lift the heels as much on the recovery going into the catch and I could get a quicker engagement in the midfoot through heel on the drive. I also found that my ankles and knees were more comfortable with the heel because otherwise the foot stretcher felt too vertical for me. I have a foam cushion on my C2 which puts me about 1/2" higher in the air and that changes the angle so maybe thats it. I am able to get my legs flat even with having the heel so I don't feel I am losing anything on the drive.
-Some do not like heeled shoes on the erg because depending on your leg dimensions, you may not be making the most of the leg drive, your legs might not be fully extended horizontal when the heel comes down fully if you are wearing a shoe with a heel, but for me they are, but maybe thats because of my foam cushion.
-Femur to tibia length ratios might play a part in what shoes work for you.
-You could try rowing barefoot to simulate a flat shoe.
This humorous video by Cameron Buchan can help with shoes, he prefers flatter shoes.
https://www.youtube.com/watch?v=ZfeM2Xu6jt4
Also I personally recommend slides if you want to do a lot of distance per week, I seem to recover better from slide workouts as they seem a lot easier on the body, so I really like them but it took some getting used to.
M36|5'8"/173CM|146lb/66KG|LWT|MHR 192|RHR 42|2020: 5K 18:52.9 (@1:53.2/500)|C2-D+Slides+EndureRow Seat+NSI Minicell Foam