Stretched Tire Advice for HDPE

1st post and looking for tire advice after reading the post quality here vs other forums. I'm exploring going down 5 in profile to use a Extreme perf tire to get more grip.

I bought my first performance car a used BMW AH3 (335i-ish N55)! Its a daily, but im looking to do 5+ events a year. So far I've done 2x AutoX and a track day and am looking to upgrade the stock tire after a gouge from a rim strike, probably from rumble strip or slalom. 

Upgrading the OEM MPS4 AS 225/45 18x8, I bought 4 Yokohama ADVAN NEOVA AD09 XL 225/40 (lower aspect by 5). Outer diameter drops ~1 inch (−3.41%)) so 0.5in lowered because they don't make a 45. Took to a Pet Boys and they didnt want to mount, said NOT OEM spec, wanted to sell me another set of 45s. Any truth to this? 

• Rim Damage — did anyone see a noticeable increase in rim dents or sidewall damage after switching to 40 profile on 8" rims? I figure the ride will be less comfortable.

• Speedo / odometer/ shift points / gearing / electronics — I calculate a ~3.4% speedo over-read. I can reset the computers shift points, unsure about OD tire size. Any gotchas with DSC/ABS?

• Other Suspension / wear — did you need camber/toe changes post-swap to prevent accelerated inner/outer wear? What alignment specs worked for you?

Thanks for the insights.

- Gripper

Most chain tire shops won't mount a non-spec tire on a car.

After fighting that several times, I bought my own machines and do all of my tire changes at home. 

The way I get around this is by taking the wheels and tires off the car and just taking them to the tire shop without the car...

Also, a 225 on an 8 inch rim is not stretched.  Stretch usually refers to putting like a 195 on an 8 inch wheel.

Also, on my car with my gearing, that would make the speedo 2mph high at 80mph.

In other words, do it.  I doubt you will notice any negative results.

Edit...  I think alignment changes would not be needed for such a small change in tire size.  However, going from a less sticky tire to a stickier tire (with presumably different construction as well) may require alignment changes.

Each tire seems to like something a little different to get max pace out of it.  However, it is all a compromise (especially if this car will see a lot of street miles).