Please dont worry about your dyno numbers, there are so many factors that influence them... not to mention each dyno machine can vary quite a bit.
As for your actual numbers, they are "ok"... for an automatic (on a mustang or an upgraded dynojet). I too was disappointed initially with 260.15 whp/261.49 tq (this with CAI/exhaust) and now only have 276.24 whp/292.15 tq (shorty headers, x-pipes, delete plates). Dont forget the automatic loses about 6% more hp than the manual, so a stock manual GT may be 260/265, an auto is typically closer to 240/245 whp stock.
My car is being built by James Klok of Boss Shinoda, so I am confident in my car's performance reguardless of any single set of dyno numbers. Your numbers are reasonable and I wish you best if you decide to do any further modifications.
2007 Vista Blue Auto GT/CS: Myer Racing Hood, Silverhorse louvers, Renegade graphics etc. GT500 TB, Steeda CAI, Deletes, UP, FRRP Shorties, X-pipe, 4:10, TC, JBA. Eibach Springs, Sways, Panhard, LCA, Mounts etc. on order: BBR stage II cams, heads, GT pumps, 39# injects, UCA, Relocation brackets etc etc... it never ends.