They are not twin turbos. They are sequential turbos.
Any engine bigger than 1368 will not be a FIRE engine any more. What started with the abreviation (sp?) relates to the engine family that started with the 999 and 769.
One thing all FIRE engines have in common is the head bolt pattern. Any FIRE head can be directly bolted on to every FIRE engine (not saying it would work in all configurations because 1242 head on 769 block would would have an unwanted rendezvous with the valves).
Trying to go bigger with the FIRE range of engines would result either in overlapping bores and/or pistons as long as in a steam engine.
The claim that the Fiat bottom ends are not very well balanced I can't verify having lots of experiences otherwise.
Also any FIRE engine can be mated with any FIRE gearbox.
The FIRE engine range is currently:
769
999
999 Super FIRE
1108
1242
1242 Super FIRE
1368
1368 Super FIRE