I have fishtail silencers from the Czech boys on my Square four. These have no baffles whatsoever in them so I have fitted Armours "pepperpots" in the ends of the exhaust pipes. These "pepperpots" are very similar indeed to the baffles in your silencer. The Square goes very well indeed but it is two 250cc cylinders per exhaust as opposed to one 350cc cylinder.
Given the length of the "pepperpot" and the number and diameter of holes in it I would suspect that it has a similar total flow area to that of the exhaust pipe. Clearly, the smaller holes at right angles will hamper gas flow but to quite what extent is a good question.
I would suggest fitting the silencer as it is and seeing what performance is like. Then, one day, take the silencer off and see if performance is any better. If the cops catch you without the silencer fitted, I would suggest that the difference in performance is not enough to be worthwhile.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.