hi irfan,
you are right about both parameters.
The Busy Threshold is mostly to compute the load of the RACH (BSS counter), but I guess it’s true that if the RACH is below this level, then it won’t be considered as “heard”.
The Busy Thrshold is non-changeable, as far as i remember, and is set to -117dBm.
The TA Filter is the only actual filter one can apply on the incoming RACH.
Regards