Benz,
I was unable to find many referances to him in an A-8, bute here's one
http://wp.scn.ru/en/ww2/f/451/2/10/34
(except for this quick blurp....http://www.ww2aircraft.net/forum/building-questions-tutorials-guidebooks/heinz-bar-s-fw-190a-7-rot-23-a-15358.html & the model kit that you already refered to)
The ref. books I have only have his Red 13 & Red 25 A-7s. of JG-1 and his Red 13 Me-262 from EJG-2.
Appears that if he did use the -8, it was possibly as a backup/replacement to his usual mount.
Is there an obvious physical differance between the -7 & -8? (regarding that I did locate this....differences are explained about 1/4 of the way down.....http://hsfeatures.com/features04/fw190a7cw_1.htm )
Not an expert by any means, but I hope this helps.
, Dave
"Yesterday is history, Tomorrow a mystery, but Today is a gift. That is why it is called the "present".