Most of the Hot Tubs I have wired required # 6 cu conductors,as per manufacturer instructions. How hot are the attics in your area? You will need to adjust your amps. Also you cannot sleeve NM outdoors, it is still a wet location,even above grade.
334.80: NM must be used at the 60* ampacity which is 40A for #8 and 55A for #6. If you run thhn in conduit then the ampacity will be 50A and 65A respectively if the terminations are marked 75*. I'm assuming that no other derating will be necessary.
If you are running it in the attic (typical here in NY for the summer is about 120 degrees F), that would take a multiplier of .75 times 65 amp (for 6 AWG) which is now rated for 48.75 amps.
The attics here in AZ do get hot, probably more than 120 deg. The spa calls for a 50 amp circuit. Derating the #6 might put it close. It would not make sense to do a splice just to run THHN through the pipe in that short run too the disc from the attic.
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