Nobody has really addressed handling qualities for IFR, so I might as well give it a go. "Fools rush in..."
I've got 367 hours in my IFR-equipped RV-9A and fly it IFR. Combined time in my RV-8 and RV-8A was 135 hours, neither one flown IFR although the -8 at one point before I bought it was IFR equipped. (Also 355 RV-4 hours). It's been a while since I sold the -8, so...
1. Hand flying in only very slightly choppy air, the -9 will deviate less from intended flight path and will require less attention to hand fly precisely.
2. Inside a cumulus cloud, even one with a vertical development of only three or four thousand feet, the RV-9A will take you for a *ride*, with serious pitch and roll deviations. I don't have comparables for a -8, but I think it would be better. Solution in the -9A is to slow way down.
3. The -8/A, as an aerobatic airplane, will respond more quickly to small control inputs than the -9. For a while, the Van's website said that the -9 handling was more "laid back..." They were quoting a pilot report that I wrote. I'd like to express this in pole/zero terminology to be more descriptive, but I took those classes nearly 50 years ago. Sigh...
4. As previously discussed in other posts, the -9 has an awful ride in bumpy air out west. Although I flew the -8 and -8A on very long X-Cs to bring them home, I don't know that I ever encountered comparable weather, so no real comparison. They were flown for fun in VMC, whereas the -9A has been used more on deliberate trips with different go/no go criteria.
As for flying either one IFR:
1. Years ago, a multi-RV, 6,000 hour RV pilot said that RVs were fine IFR if you never had to do any paperwork, like look at a chart or write a clearance. This was back in the RV-4/6 days.
2. An autopilot really makes life easier for IFR in an RV, especially if you have a glass cockpit and are messing around with all the various screens. Then again, an autopilot is not a panacea as you need to monitor it closely to make sure that it's really doing what you thought you asked it to do.
3. Even if you have an autopilot, you need to be able to hand fly IFR, just in case.
And a last comment on what other pilots will state. And by way of background, I've published something over 100 pilot reports and at one point thought I had my act together for handling qualities evaluations:
1. There is rampant dysfunctionality when pilots discuss their own airplanes. Almost nobody can make a distinction between "I really like the ownership experience I've had" and "This plane has great handling qualities from an objective, engineering point of view."
2. Pilots are highly adaptable and most pilots quickly learn to fly whatever they're in, regardless of handling qualities.
3. Very few pilots are aware of handling qualities. I've flown with a number of factory demo pilots who were excellent sticks but had absolutely no idea how the plane handled objectively or, in one case, of what techniques they used flying the plane.
So what's the best advice?
1. When I was working on my ATP, my instructor said, "Instrument flying is war." In other words, bring your A game, all the time, and if you don't have an A game flying IFR in an RV...
2. There's a good chance that if you fly both the -8 and the -9, you won't be able to tell the difference.
3. Frankly, I don't know how much real world difference there would be between the two. RV-9A for handling qualities, RV-8 possibly for ride in turbulence, RV-9A for more cockpit and panel space and tricycle gear makes it easier to land in an ugly crosswind when you're beat up after a day of hard flying. (Yes, you said -9 and not -9A).
4. To thine own self be true. There will always be other pilots who can easily do things that aren't easy for you and will misuse that personal skills assessment as a measure of the airplane.
Make your best decision and either live with that decision or sell the plane. In my case, that decision is that I'm very circumspect in what weather I'll fly in, VFR or IFR, considering ride quality/fatigue as major factors.