Can the texture suffer from too long in cure? Not sure, but again I don't think so, kinda one of the main reasons to cure in the first place.
I asked Kevin about this and his answer was yes, curing too long can make it tough.
I made another attempt at this and smoked it yesterday, another 12lb flat. Brine cured and pumped for 72 hours. I smoked it sub 200ºF for about 12 hours and it was 140ºF internal when I removed it.
I then steamed the whole brisket, checking hourly for tenderness. It started feeling more tender about 4.5 hours in, about 180ºF internal. In total I steamed it for 6 hours.
The results were significantly more tender than previous attempts, the color was improved (looking for red here rather than pink) on the down side the product was somewhat dry and as you might expect it lost a lot of flavor in the 6 hour steam bath.
It didn't get as tender as non-cured brisket can so I have to keep that in mind when checking for tender. This one loosened up a bit and felt more tender but by the end it still didn't feel as tender as non-cured brisket can.
Conclusion from this trial, to try next time is smoke hotter, like 250ºF - 275ºF foil around 160ºF and carry on at the same temp, start checking for tender around 180ºF (settle for improved tenderness, can't seem to get 'probe sliding into room temp butter' feel with cured brisket). That should take the place of the steam bath, help it retain more flavor and I think the higher finish temp is responsible for the improved color in the meat.
Inching closer to perfection.
Thanks again to Kevin Kruger for his valuable input.