The 540 giveth, and the 540 taketh away.
Flown solo, -8 stick force becomes heavier and heavier as the CG moves forward. An angle valve (IO360 or 390) with a metal prop is not as fun for solo yank and bank as the 360 parallel valve and light prop combinations. I have not flown a 540-powered -8, but the trend is clear. Trimmed for 150 knots, a pitch pull to 3.5G is likely to be quite heavy, just like the angle valve fours. It's one of the few things I don't like about my current ride.
Above, Rob is reporting an empty weight roughly 100 lbs more than average. The extra mass is engine and lead in the tail, so it will have a higher polar moment of inertia for the same weight and CG. That slows velocity change in pitch and yaw, most notably spin recovery.
If we assume equal loaded weight, steady state climb with the 540 will be superior. It's the thing HP does best. Super-8 climb should be a real hoot on a cold day.
Cruise performance is best compared using a speed vs fuel flow metric. I would expect a Super 8 to be worse than any four-cylinder version when flying a typical solo cross country, a function of wing loading. RV's are slower when loaded.
Super 8 top speed should be higher, although not by a lot as compared to the big bore fours. Again it would exhibit higher drag due to loading, plus the power squared rule applies.
The angle valve fours and the 540 both result in stable haulers. It's the plus side of the forward CG trade; they fly nice with the rear seat and baggage full. However, apples to apples the Super 8 would be payload limited to around 100 less lbs. Listing a 2000 lb gross weight on the paperwork doesn't change the spar.
The 540 costs more to buy and maintain, although an owner can probably recover the purchase difference at sale time.
Quick comparison. Flown solo, my 390 fastback will exceed VNE down low (best recorded was 206 KTAS), and get close at 8000. At 11.5 GPH and 6500, it will push 190 KTAS (218 mph). After some tweaking, the typical 8~10K cruise is currently 182 KTAS (209 mph) on roughly 9.1 GPH. Empty weight is 1167 with a metal Hartzell. There are no speed mods other than cooling drag reduction, the fastback, and wheel pant boots. The 390 is internally stock.
What follows is pure opinion, based on the above. I'd recommend a Rocket if going fast is the primary goal, an angle valve 8 for a hauler mission, and a 180 HP parallel valve 8 for a fun machine. In a hard comparison, I don't think a Super 8 offers enough extra to be worth the compromises.