I purchased a 2013 SX192 just before Memorial Day. Like everyone else, my boat suffered heat soak within minutes with the stock intercooler. I quickly purchased a NOS SVHO intercooler from Riva Motorsports, installed it, and ran it like I stole it. This cured my heat soak issues nicely.
Regarding fuel octane, on all of my boats I have always fed them 91 octane non-oxygenated fuel. My SX192 was no different and the boat ran just fine with it.
After reading the forums here (and the Yamaha owners manual) I ran a full tank of 87 octane gas with 10% ethanol. After 20 minutes of running at 6000 rpm (not quite WOT), the dreaded heat soak was back and I lost 500 rpm and could barely get above 40 mph. I was so frustrated! I thought I was going to save some cash on fuel by using the 87 octane but to no avail.
Just yesterday, I filled up again with 91 octane, non-oxy fuel and headed for the lake. The entire day and 3/4 of a tank of fuel burn later, I am happy to report that I had zero heat soak issues once again. That settled it for me - I will only use 91 octane, non-oxygenated fuel in my 2013 SX192!
On all three days of testing I had almost identical atmospheric conditions; 85 degree air temp, 79 degree water temp, and 5-10 mph wind. Payload consisted of full fuel load (30 gallons), 2 adults, 2 children, 25 liter cooler filled with water and ice.
Everything on the boat is bone stock except the SVHO intercooler with 2 dedicated discharge lines.