I use snaps to attach all my lures as it allows me to quickly change from one lure style or color to another. It is possible that taking off the split ring and attaching directly to the lure will create a different action, but I rarely do so. For me it is a matter of efficiency. You mentioned in your question that you had difficulty getting a snap on a #8 Shad Rap and I concur. The Shad Rap eyelet is set very low in the lip — for good reason. The lures that have high eyelets tend to have wilder actions, but are also very difficult to tune, as well as, maintain tuning.
My advice would be give it a shot as an experiment and see if attaching directly increases your odds. My experience tells me the hassle of trying to fiddle with direct attachment is not worth the effort — in general.
You also asked about quick clips and again my experience is to use a standard duolock snap for crankbaits. As I mentioned above, I am never against experimenting with things that make life easier, but I have not found these clips to help me that much. I think mainly because I can tie knots quickly with my eyes closed. On the flip side, if you are like my father and it takes you 10 minutes to tie a knot (he’s blind as a bat) these clips might save you some time and could be worth a shot.
Hope that helps.