There's a few reasons why you are not getting the air flow/power you are seeking on the top end. I'll tell you what I did to my .30 caliber to run 980 FPS @ 170 bar, and you can do the same with your .25 caliber and probably be in the 1000fps range. Port and polish the housing, port the block, port the transfer port, open up the pellet probe hole to match the other three port sizes, add a brass hammer weight, open up the valve seat between 6-6.5mm, and turn up the regulator. The long story:
To get the "total package" out of opening up the brass transfer port you also need to open up the block's port, as well as the valve housing. All three ports should be matching when assembled so there is not and steps or ledges for the air to flow around. Also, you need to be sure that the pellet probe is in the correct spot when pushed into the transfer port. (Your goal is smto nehave one smooth transition of air from the valve seat out of the barrel.). Check out this thread I did on porting the valve housing. See the nice raduised corners and how I ramped the back of the housing to smooth the airflow?
https://www.airgunnation.com/topic/fx-impact-valve-housing-modifacations/ The seat stock valve seat can be opened up to close to 6.5mm, but it makes the back of it weak and may cause the thing to break and leak air. Then you'll have to just buy another one, or make one. I modified the stock seat by putting it in a drill and filed the center out slowly with a small round file untill it was as big as I dared go (6.5mm) without cutting into the o-ring groove. Then I polished the seat and rounded the leading edge of the seat to the air moves in around a nice raduised corner once the valve seal is opened.
Also, get a brass hammer spring weight as well. When using the higher regulator pressure and to open up the valve stem quickly and to hold it open a touch longer, you need more weight.
There's a kit available for the Impact's on eBay made by forum member Edosan that has the bigger valve seat, hammer weight, and a delrin C3 bumper. This kit and the porting job will get you all the power the gun can handle. Shot count will go down considerably though untill you add plenum tube space.
I did not make much more plenum space, aside from modifying the valve housing to remove excess material. The plenum space seems to only allow for more air for higher shot count per fill. It may add little bit of power on the top end if you're running a .30 caliber with a regulator pressure at or above 170 bar, but I will only be able to confirm that for sure when I add more space and I obviously don't need it to push these 50.15 grain JSB's at 980fps!