I shot 4 x 15 round matches with the Izzy at 5 yards, open sights, 2-hand grip at each of 4 targets types:
10-Meter ISSF 5-bull target, 3 shots per bull - 65
Steroid 15 bull target, 1 shot per bull - 100
Gamo 5-bull target, 3 shots per bull - 98
Glock 4-bull target, 4,4,4,3 shots per bull - 137
The Glock target bulls and rings are exactly the same size as the Gamo target bulls and rings. So, with a black on white bull vs a white on black bull, my score went from 98 to 137.
The black bulls of the Steroid 15 targets are the same size as the Gamo bulls, but the rings are much smaller, and anything out of the black (10,9,8,7) bull (ie, an 8 or less on the Gamo or Glock targets) scores zero. No surprise that the Steroid target score is low. All 10 rings of the 10-meter ISSF target fit inside the 7 ring of the Gamo target.
If I over-lay the Steroid holes on the Gamo targets, I would have scored 137. Overlay the 10-meter target holes on the Gamo, and I would have scored 131. It's all relative for scoring -- just different scales.
Small sample size, yes, but I feel pretty confident in saying that shooting a black bull on a white background is THE preferred target for open pistol sights. Scoped, probably not a lot of difference. If you want to keep the scores more encouraging (a lot fewer zeroes), shoot the Gamo or Glock targets. But if you want to make it hard to compete with open sights, at least for me, choose the Gamo white on black bull or the square targets with 1 cm bulls.
May I suggest that we may shoot EITHER the Gamo-5 or the Gloc-4 bull target for the open pistol match, depending on whether one prefers a white center bull or a black center bull? The target sizes are the same. That said, I will make the same suggestion for the open rifle matches using the Gamo-5 bulls. Let those who shoot diopter sights and prefer a black center bull shoot the Glock target rather than the Gamo.
