I think you have hit on the most likely reason for the length of the 1070's development period: it is a new design, unlike the 990 (which is a modification of an existing piece of hardware). This means there's more hardware to develop (including a amp section, which the 990 doesn't have at all), plus more software development to deal with (although, based on the P-965 manual and comments from a couple P-965 owners, it sounds like Outlaw had a fair bit of the P-965's code revised or re-written).

Ah, speculation... If the 1070 sounded better as a pre/pro, I suspect that Outlaw would happily sell a lot of 1070's.