Can someone fact check this for me?

So I needed to replace the rear tires on the 2007 MS3, someone had at some point downsized to 17s, which I'm happy with, but they also downsized the overall tire diameter by a few points. Not a lot, but a bit. 2.4% or .6" smaller diameter.

When I replaced the rear tires, I wanted to get close to stock in the anticipation that I will tear the front tires off at some point. 

So now the rear tires are 215/50/17 and the fronts are 225/45/17. 25" vs 25.5" tall, 2%. 

I didn't think this would be a big deal, and I'm not sure it is, but can someone confirm this? 

I'm asking as I was out driving yesterday and became abruptly aware of an extremely large amount of oversteer on a gravel road and am wondering if it's the tire size selection, or if its the 28mm RSB up from OEM 26mm. This is my first MS3, and I didn't install the uprated RSB so I have no point of reference. 

Might be just the twitchy nature of the chassis, but if I was to let it, I think the back end would happily step out at low speeds with a little braking and steering English. It was odd to experience, but my last car was a certifiable land yacht so I'm not sure if this is to be expected. 

If need be, I'll swap tires F to R and see if the behavior persists, and if all else fails, replace the fronts sooner than I anticipated. 

Thanks y'all!

Stiff sway bars on loose pavement are a bad combo in general. The tire swap idea is a good idea in general , but I doubt the new tires are any worse than what you removed on gravel. Any front wheel drive car will put the tail out with trail braking on a slippery surface, and that is magnified by stiff suspension. Even stock the MS3 is pretty stiff for gravel. 

Your should be fine. Most people in the tire industry want to keep the sizes within 3% of each other on two wheel drive vehicles. All wheel drive vehicles can be pickier. Definitely keep them the same size side to side. 

You might have a wheel alignment issue causing your handling issues. Many shops will check that for free (hoping to do the work to fix it if it needs fixing). I'd try that before rotating the tires.

I have no experience with that specific platform but a larger rear bar will free up the car as will having wider front tires than rear and having the rear of the car sit higher than the front.  The tire differences are pretty minimal but you've got over 1/8" of rake with the tires you're running and on some cars that will loosen them up noticeably.

I'd try swapping the tires front to back and see if it helps.

I wouldn't expect to see a radical change from that size difference. I don't know what the stability control setup is on a 2007 MS3; you're not seeing any lights indicating the car's unhappy about the disparate wheel speeds, are you?

My main thought is that your big moment was on gravel; beyond that you sort of observe that it feels like it might get out of shape if you let it, but I'd say that "a little braking and steering English" is more like provoking it than letting it?

Mostly I suspect I just don't have a sense of the symptoms from those data points. I'd be curious about alignment, and if it's really behaving oddly I'd be looking for an explanation outside of that tire size difference, whether that's tire condition, brakes, bushings... That rear bar, if the active length, arm lengths, and mounts/links are the same, should be about 35% stiffer than the stocker, and that is going to make it more tail-happy. But it sounds like we're both trying to understand changes from your tire swap AND getting to know this car at the same time, so we don't have a lot of baseline to say "this part's the car, and this part's the tires..."

Reasonable?

I can say that ABS systems don't like staggered tire diameter all that much. 

When I had that much difference front to back on my Colorado it would kick the cruise off when I'd go around a bend. I wouldn't expect it to affect the handling 

Ms3s (particularly  first gen ) are prone to off throttle oversteer from stock, especially with an uprated rear bar. I know mine has tried to swap ends on me more than once in autox situations and I was running a lot more rubber and bar than that. Even stock they're pretty twitchy. 

Small update to this. I went through and did sway bar bushings and links, only to find out that the rear bar is a Corksport bar (I knew this), but what I didn't know was that it was on the firmest setting. 

Whoopsie

I move it to the softer of the two and it is much, much better!