My solution
Following what Ron Schreck posted a number of years ago and consulting with other RV-8 aerobatic types, I elected to add 20 pounds of ballast under the vertical stab. I added 5 lb increments and reached 20 lbs. and have spent the last year averaging four aerobatic flights a week with never an issue.
My RV-8 is nose heavy,Hartzell constant speed,IO-375 with a full Raven inverted system. I installed a smoke system 5 gal tank on the lower aft baggage shelf, but rarely carry more than a gallon of smoke oil. I generally fly with 22 gallons, 11 aside, having a Flop tube in the right tank. Without the ballast, the aircraft was extremely nose heavy at 70kias on final. The day I added the ballast, I had a new aircraft. I use a free app called CG finder, and depending on fuel load I can put a 170lb pax in the aft seat and still be within the c.g. Range, but I never fly aerobatics with a passenger. I did replace the heavy Odyssey battery on the firewall with an Earth X Lithium. The aircraft flies all Sportsman maneuvers just fine. The only limitation being the pilot as to the quality of maneuvers.
My takeoff weight in this configuration is 1544 lbs. My takeoff cg is 82.17” and zero fuel cg is 82.37. My measured arm for ballast placement is 232.38”.