I did some tests comparing a pimped RIGOL DG811 (DG992-mode) vs. a pimped Siglent SDG2042X (SDG2122X-mode).
I used the same BNC cable which I unpluged and repluged during the tests. The mesurement was taken by a RIGOL MSO5000 oscilloscope. BNC cable was terminated using 50 ohm on T-adaptor.
Rigol DG811 (pimped to DG992) set to 25 MHz square wave, 4 Vpp, 50 ohm:
Siglent SDG2042 (pimped to SDG2122X), set to 25 MHz square wave, 4 Vpp, 50 ohm:
Comparison of both signals, Siglent's trace in green colour:
Now sine wave, 100 MHz, 1 Vpp (since 1Vpp is max. amplitute of DG992), 50 Ohm.
DG811 (pimped to DG992):
SDG2042X (pimped to SDG2122X):
Comparison of both traces, Siglent in green colour:
You can clearly see that the RIGOL is much more attenuated at 100MHz than the Siglent. Furthermore Siglent's square wave is a little bit better. But SDG2042X costs 565€ (upgradeable to SDG2122X) compared to 236€ for DG811 (upgradeable to DG992). Considering the price, DG811/DG992 is the winner in my opinion.
Update: I changed the labeling of the measurements because of TurboTom's objection. I do not own a genuine DG992, nor do I have access to one. Sorry for any confusion.