What was your ripple?I'd pick a 4070 over the castle 1717 all day. I think an mmx8s is more than enough for a 4070 geared for 100 mph. My Limitless Jota is geared for 80 mph on 6s. I've only tried it on 8s once and I didn't have any problems and 8s made it 107 mph.
If you want to bash on 8s like that I think it really depends on how you drive. I think motor temps would be more worrisome than esc temps. I use a 3s battery connected to 12v fan on my tp 4070 cm 2200kv so I don't have any problems with motor temps.
You could also go lower kv, I think the 1700kv would do well for the task.