Commodore 900 vs Commodore Amiga 1000

[toRus]

Commodore were a really lame company and did not do Amiga justice most of the time. Despite that, they did the wise thing of staying out of the 80s business trend of trying to captitalise on the aftermath of the AT&T split and revive a terrible (for the desktop) OS. In the end of the day their only chance would be to sell el-cheapo Z8000-based machines to the enterprise (Z8000 was introduced in 1979) rather than risking with a new product (Z80000) of a dieing company (Zilog was in rebuilding/restructuring/survival mode under Exxon). Commodore would fail miserably as it failed with the Commodore PC later on and ultimately with the Amiga since they did not have the marketing to compete in either the enterprise or the home market nor did they have the vision to understand what they really had in their hands (the Amiga).

The Amiga, even though being underfunded and underdeveloped under Commodore, became an economic and cultural success since the hardware and software was too good to fail and the word of mouth prevailed over any lame marketing efforts of the parent company. But in enterprise you have to market yourself in order to sell the product.

[toRus]

which is the same year MC68000 debuted

Yes. MC68000 was a more capable processor and 68020 was released 2 years before the Z80000 - the latter was good on paper but got delayed and did not sell.

[toRus]

Zilog's Z8000 had some interesting features but also bugs, compatibility issues and it was not ready from the start (not the MMU). It didn't translate good for the desktop either. Olivetti was selling $5000+ HD-less and unixless Z8000-based machines then; it was the HD, the RAM, the support that was expensive. The Z80000 was only on paper and the company under Exxon was a mess and many people had left in the mid 80s. This was not the case of 70s with CBM buying up and coming MOS, it was a case of entertaining opportunities to enter the enterprise after the party was over. It would take some time to catch up and find clients, the trend in the CPUs had already changed, it was not a sure thing they would manage to license Z8000 to other (i.e. Olivetti) and it could take years before Z80000 was ready. You never know but I guess they would fail miserably.

[toRus]

I managed to find a Sieve benchmark of Z8001 vs some other CPUs of the time...

http://www.atarimagazines.com/v4n6/STperformancetest.html

According to this, a 5,5 MHz Z8001 running Unix was 2 times faster than a ST with a 8 MHz 68000 in this particular benchmark.
So a 10 MHz Z8001 would give a very solid performance in a C900

That's not a good benchmark. It compares different thnigs at the same time. And certainly not indicative of CPU performance, unless you are ok seeing the same CPU (68k) performing 3 times faster/slower using different OSes and compilers.

[toRus]

Don't get confused with the Z80180 which was basically an imroved Z80. Z-88000 was only in test phase after being late 2-3 years, the bugs were not ironed out and by 1987 the word was that it did not perform very well in practice and was considered vapourware. I don't think CBM would fair better than Exxon and I find it difficult to believe that a C900 machine would stand a chance against Amiga in the desktop or the existing minicomputer market in the enterprise.