TexRail FLIRTs can not reach 100 mph.
TexRail will rarely, if ever, run faster than 60 mph (96 kph) in normal operations. The DMU FLIRTs TexRail bought per Stadler specifications could reach a maximum speed 79 mph (130 kph).
The best answer to your question comparing the specifications differences between an EMU and DMU versions of the FLIRT, this Stadler link for Greater Anglica trains have both- in 3 car and 4 car lengths.
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TexRail uses 4 cars , so let's look at the 4 car version in detail.
Max acceleration electric (0-40 mph) = 1.1 m/s2 (3.6 ft/s2) or 2.45 mph/s
Max acceleration diesel (0-40 mph) = 0.9 m/s2 (2.9 ft/s2) or 1.98 mph/s
Max speed 100 mph
Max electric power at wheels was the same for both the diesel and electric versions, 2600 kW
Therefore, possibly completely wrong algebra - but here I go anyways....
Electric = 40 mph / 2.45 mph/s = 16.3 seconds
Diesel = 40 mph / 1.98 mph/s = 20.2 seconds
I have no idea what the acceleration curves are above 40 mph, so I can't accurately calculate what the time to 100 mph would be. Assuming the same acceleration curve to 40 mph, yes I know a very bad assumption to make but fun never-the-less:
Electric = 100 mph / 2.45 mph/s = 40.8 seconds
Diesel = 100 mph / 1.98 mph/s = 50.5 seconds
So, the diesel should only take an extra 10 seconds or so to reach 100 mph than the electric.