I too tried replacing the main centre silencers with straight-through pipes, keeping the original BSA C15-type resonators that I actually really like.
I found that the noise was excessive and frankly anti-social, and detracted from my pleasure in driving the car - that from someone who thinks Contis on bevel-twin Ducatis are OK, and megaphones better.
Furthermore, with the greatly reduced weight there came a whole new spread of harmonics and vibrations from underneath the car that I suspect would require a completely different design and elasticity of rubber-blocks to cope with.
So after 50 km I took the carefully TIGged and bracketed assembly off, and chucked it in a corner ..........
I then re-examined the silencers I'd thought I could do without and discovered that there was a blanking or baffle-plate about 4" down from the start of the inlets, that diverted the gases sideways through a perforated tube ; oh well, what the hell, I thought, and got out the 45mm hole-saw, like you would.
Being quite deep it required a Heath-Robinson assembly of 1/4" extension and 9mm socket, see picture, but being well guided it cut quite nicely and the bore just needed finishing off with the die-grinder.
Each silencer yielded a big penny washer, then, and suddenly you could see the outlet - the new hole turns them into motorcycle-type absorption silencers and short-circuits the baffling.
I show the picture of the otherwise unremarkable and unattractive items in question just in case it helps members identify theirs as being potentially the same ; no idea what make they are, they were fitted by the previous restorer some time in the early 90's.
The result is Exactly What I Wanted - enough engine vroom from within the cockpit on steady throttle low-rpm, without offending passers-by, a much sportier and healthier sound when accelerating, and an exhaust note that is 'present' at high speeds without detracting from the luxury GT feel. I fancy the car actually feels quicker but this may just be The Big Dick Factor.
As a final observation, few details have made as much difference to my enjoyment of the car as being able to silence all the tiny chuffing sounds emanating from the exhaust connections ; a Clarke pipe expander and a can of GunGum, together with decent 360? pipe-clamps were the answer.
