A quick search did not lead to conclusive results about the acceleration of diesel FLIRTs (Stadler used to have way more data sheets around and available … curse them for no longer making these).

The only direct reference I found was about the order for Estonia, where the electric trains have an acceleration of 1.2 m/s2, which is considered to be "excellent". The acceleration of the diesel trains was also declared to be "excellent".

In theory, the initial acceleration should be similar, because the tractive force is. However, because of the limited installed power of the diesel engines, the maximum tractive force can be maintained only to a lower speed. This means that the acceleration to any speed above that value will be lower.

This actually led to the idea to replace one of the 4 engine bays with a battery pack. This has the advantage that regenerative braking is possible (charge the battery), and the battery will also serve as a booster for the acceleration (which then would be comparable to the all electric train, but still over a smaller speed range.