The talk about E-10 gas with less HP is relatively insignificant when talking about an 8 hp snowblower engine. E-10 has about 96% of the BTU of E-0, meaning your engine is 7.68 hp which will not be noticeable. Also note that if you're running E-0, you should be running a small amount of isopropyl alcohol to absorb any moisture in the fuel. Often times, the E-10 will bring the carb jetting closer to correct and actually result in a net gain of output power. Ask snowmobilers from the days of carburetors how E-10 could be a tuning tool on a warmer day. Bottom deal- don't get too hung up on your gasoline's octane or E-10.
Regarding the OP's D-24.. A couple of checks- this might be a dumb question, but are you twisting the throttle knob completely to full position? The engine will run full rpm with a small amount of rotation, but not pull a full load. A year ago, I thought my D-28 was totally underpowered at first. There may have been a break in situation or I may have not been turning the throttle knob all the way. Very possible I wasn't twisting the throttle the full turn at the time. Whichever the case, ample power now. Not crazy extra power, but ample to get the job done.
Interesting that you found the drive pulley's retaining screw to be loose. I found that same screw laying at the bottom of my service cover last spring. Another member on this site also reported the same condition.
Hope this helps.