there is one oryginal 2 channel owon that is set to be 200mhz bandwidth. but it is still 1gs/s and still only 10k samples buffer. i wonder how a triangular wave would look on it at those conditions, and if it's firmware could hack lower spec models because they limit the number of samples in scopes with bandwidth lower than 50mhz quite drastically.
(a) 200 MHz bandwidth in a
two-channel scope with 1GSa/s is alright. You get 500 MSa/s in dual-channel operations, so the Nyquist criterion is still met -- although it's a bit marginal. If you feed a 200 MHz triangular wave into a scope with 200 MHz bandwidth, it will look like a 200 MHz
sine wave, because none of the higher harmonics make it though the low-pass filter. That's actually just determined by the input bandwidth, not the sampling rate.
(b) Not sure I fully understand your idea with the firmware hack. But if OWON sells a lower-spec model with lower
sampling rate, it is very unlikely that its ADC and acquisition hardware (probably an FPGA) can be run at 1 GSa/s. Again, fast ADCs cost actual money. OWON would use a cheaper, slower ADC in the cheaper scope.
It is quite common for scope manufacturers to offer a few related models with different
bandwidth specifications which are based on the same hardware. But I have not come across a family of scopes with different
sampling rate specifications while being based on the same hardware.