A question about estimating non-filtered data
Well you could apply an inverse filter, but with a third order roll off having been applied half a decade below where you wanted it you will be looking at 30dB of gain by the time you get to the sampling limit at 100Hz, which will bring the noise up by 30dB.... 

I doubt you are going to get anything useful past maybe 50Hz or so, and it seems like a lot of faffing about if you can just re run the measurement. 

It very much depends on what the data set is, and how important the 'high frequency' content is. 

Time to take that amplifier and do some frequency sweeps and FFTs to get the gain and phase response of that filter (And you really want it to be that particular unit), then fire up matlab and start playing.