Hi
When you use 4-(G)FSK modulation, preamble and sync word are sent in 2-(G)FSK (see Figure 9 in the users guide). Taking this into account the new time measurement will be:
7 bytes @ 2FSK + 28 bytes @ 4FSK = 11.2 ms + 22.4 ms = 33.6 ms
Adding around 500 us of IDLE to TX time gives us about 34.1 ms.
Suspecting a small delay for the LED drivers the total time will most likely add up to approx 35 ms.