I live in dense urban area in center Poland, near 4 BTS' and with FA filter installed I have worse reception, less frames/s and max range is decreased about 10-15%.
EDIT: Filters from 925 MHz, so there is no GSM Downlink filtering at all (declared is 980 MHz).
If the close-in energy you have is at a known fixed frequency, you can try cutting a quarter wave stub to filter out that frequency.
The bandwidth of a quarter wave stub is about 20% of the frequency.
You put a "T" fitting in the line from the antenna to your SDR, with the stub hanging off the "T." Leave the end of the stub open. The quarter wavelength calculation needs to include the velocity factor (Vf) of the cable used (usually between 0.66 and 0.80). The traditional approach is to calculate, cut a bit long, and then go through steps of measuring performance (with one of the SDR power tools) and trimming. Plan on making a second stub as you trim too much on the first one! As an example, a quarter wave at 925 MHz with a Vf of 0.66 is around 54mm -- from the centre of the main conductor to the end of the stub. A Vf of 0.8 gives a stub length of about 65mm.
Oh, use the FA filter as well: Antenna --> "T" with stub --> FA filter --> SDR. The insertion loss of the FA filter is reasonably low.
If the stub + FA filter doesn't do it, then you need to go to the next level -- a cavity filter or a SAW. Cavity filters are bulky, expensive, and have very good performance. SAW filters are inexpensive (around $20 US on eBay sourced from Eastern Europe), have very good performance, and about 2.5 dB insertion loss. I did a number of posts on hand-assembling SAW filters for ADS-B use, and ran them in my systems until the arrival of FlightAware's SDRs with filters built in. I still use the FA filter in front of the SDRs, as they clean up a lot of the RF crud in the area. I also have data that demonstrates putting the FA filter in front of the filtered FA SDR improves performance by a statistically significant amount. Yeah, if I didn't waste so much damn time sleeping, a cavity filter or a stripline would be interesting to try.
Have fun, keep good data, and let us know what works and what doesn't!