The stock dwell on the GEO2 was 12.5. On very cold days I would up my dwell to 14.5 if shooting really brittle paint. While you can dwell tune as outlined in the various posts in the forum, I found too low a dwell effected my shot to shot consistency. You can underbore to bring up the fps and improve consistency but there are a variety of factors that can effect dwell tuning that are out of your control (weather, paint quality).
There is a very obvious difference shooting the gun at 12.5 versus 10.5 in kick and sound signature. Well broken in guns might be able to handle that setting but every gun is a bit different. The only times I ever had a problem with my GEO2 was when I deviated from the default settings. The Geo2 will always shoot and have the efficiency of a GEO2. If you want improvements in either area there's always the GEO3.
That being said I had my GEO2 for a while and loved it.