Don seems to give folks the maximum rating, that being with whatever get the max. His shotgun ( discovery based) gets abouve 300fpe - I believe- and naturally the rating mentioned for the marauder is not for a .177. His valve will make those interested in making a .30 marauder , even a .45 or smooth bore and if one could live with a muzzle loader it could be a very easy low cost build. And his discovery valve is unlike pretty much any other valve I have ever seen. And they are very adjustable.
SO glad I already have all the airguns a person could ever need.
Don does do excellent work, I've seen a lot of his work and spoken with a lot of other customers of his, never a complaint.
John