I keep an eye out for any other Z4's I see out and about. From memory there are easily 5 or 6 that I would class as local to myself who I refer to as my rivals 8) . Most of them are 20i's, I saw one 28i in a car park once and I can think of 2 maybe 3 23i's I've seen (Which is what I have as well).
My other observations are that most are not m-sport trim, they seem to be driven mostly by men, though I've seen a few women as well. Most Z4's I see are white or silver and one had an orange body and a black roof. Most of the drivers are middle aged 40-60. I saw one younger guy like myself driving one (Though he didn't share my enthusiasm at seeing another Z4 lol).
As for the new Z4 I've seen 2 of those on the road and both were blue 20i. The original soft top Z4 I rarely see ever these days.
With regards to the 3 litre engines they cost more than the others new and most people probably preferred to use that extra money saved to spec the cheaper car out with extras so there are less out there as a result would be my thoughts.