The good thing about SR and iRating is that once you reach whatever level you aspire to, they don't usually mean spit. Once you buy the cars you want, SR really means nothing... absolutely nothing. You can see that in the upper splits in the higher license series. There are plenty of Kyle Bush emulators. They don't come and go either. They're there for however they wish.
As for iRating, it gets harder to change the longer you race. And affects nothing that affects your choices when it comes to buying new cars or tracks. It's actually easy to change if you choose to "win farm" or tank it in hopes of having slower competition to race against, but I'm assuming you aren't that type.
As for iRating, you'll probably be fairly locked into a number range after some months that probably won't go up or down unless your skills go up or down. You'll find if running a popular series you'll settle out and stay in about the same split for long periods of time.
In a couple of my favorite series, I've discovered that I'm almost guaranteed to be in the top split for sure and over half the entry will be aliens.
Guaranteed... and it's been that way for over a year and a half. The idea that losing will put you into lower splits where you will win sounds great but for most, it isn't reality.