I spent half of year to research spacer matter.
First I talked to Honda Engineer of Car Design Team (my best friend).
I went to talked to mechanic. Finally I looked at dealer's chief mechanic's car, a few of them own M3.
Result is as following;
Spacer place in between wheel and hub, there is alway space in between; whether it is small or large. While you drove throught curd and bump, there is a challenge to spacer.
For same material, material strength is depend on its thickness.
For thin spacer such as 1mm, you can bend it by hand
For thicker spacer make by very strong material that can hold hits while you drove throught curd and bump. Problem is force on wheel bearing, bolt length in hub is unchange, increase of spacer, increase of bolt total length.
(Spacer thickness/original bolt length)*100 relate to percentage of increase force to wheel bearing.
Every mechanical part will break, car engineer can only design part to fullfil warrantly period with given data. Changing bolt length change all geometry data.
If spacer is ok, car manufacturer will be the first one make it and put their logo on it. They can also make different thickness of hub and put their logo on and call it performance part. If they don't know how to make it, they can purchase from after market and print their logo on it.
Laterly wheel design changes to deep rim because of brake caliper. If adding spacer is a solution, they make wheel with thick spacer and long bolt. Manufacture make wheel with deep rim, such as BMW, 09 Subara STI. Changes a structural design cost a huge investment.
When I talk to senior mechanic, they said "I am happy with my OEM rim". Why???
One of mechanic told me, "Adding a spacer and long bolt, drive to Dealer and ask for wheel bearing warrantly, you will have an answer."
Finally I will change my rim only with original offset.
There RIM manufacturer do custom offset such as
http://neezinternational.com/structure.html