I have 25mm bolt on spacers on my R32 GTR - and this is was my understating is on a couple of the issues you have raised
As far as I know the law in QLD states that it is illegal to alter the overall track (width) of a vehicle by more than 25mm when fitting non-oem wheels. This applies to ALL wheels with or without spacers. Any decent mag wheel shop should be able to tell you this.
So, if the mags you fit do not have the same offset as the original Nissan wheels that came with your car, and the spacers return the overall track to within 25mm of original - they should be fine. I've had a 911 in my past that had spacers fitted in the factory ???
I've also had problems with a steering shudder at around 100kmls - but I swapped my spacers around and the problem went away. I agree, in that it's the spacer that is not balanced (buy better spacers )
I also know that fitting spacers alters the geometry of the steering – which can put additional stress on the front end - my upper control arms failed almost straight away for example.
I also get quite bad 'tram-lining' on some roads – like the freeway (car seems to follow the ruts in the road). I've been told this is also due to the spacers (SK told me this – so it MUST be true )
just my 2c worth – all opinion – little fact