Didn't realise they were produced from 1999 - 2006 and the stats on that S62 engine look stronger than the S54.
With a worldwide production volume of just under 6k it looks like they only produced around 300 more units than the Z4M roadster.
The following is from Wikipedia regarding the hardtop:
Every car was shipped with a colour-matching metal hardtop with a rear defroster. Unlike many contemporary hardtops, which are provided for practical rather than stylistic considerations, the hardtop of the Z8 was designed from the outset to complement the lines of the car's styling.