Ok I tried the 7s setup this morning and I must say its a more sane setup. 8s on the 18x21 is way too hot for the overall system. On 7s it takes a while for the plane to accelerate to speed but once it gets there its fast. Not insanely fast but fast. I had to hold throttle on for between 3-4 secs for the plane to get up to speed. As usual the stalled prop sound holds on longer than your typical F5b plane of 2-3 secs bursts.
Now I'm thinking should a 18x16 work better to get the plane to bite faster and accelerate better with less prop stall.
This 7s setup is 2900-3000 grams AUW.
After the last climb and 2 mins of thermalling prior to landing, pack temperatures were about 50C. Motor was about 45C. So was the esc. No over temp cut this time for about 9-10 climbs. Total motor time 40secs and packs are down 55%. It's about there I think. Would love to try a 18x19 prop.
For those who fly in cold countries. I think you could get away with the 8s setup and not over temp the esc. But you could destroy the motor first.