I flew my 6 with Vans standard 4 point for hundreds of hours over 9 years. I upgraded to the 5 point Crow harness last year. The biggest advantage is comfort when you need to tighten yourself in for turbulence. 4 point lap belts tend to "ride up". The crotch strap keeps the lap belts in position all the time. You can really crank them down and everything stays in position.
Adding the crotch strap in a 6 or 6A can be challenging. The 7/9 kit may or may not fit. I bought the kit and then ended up fabricating my own anyway. The kit was a good guide, but didn't work for me.