I'd love to know what the ballistics on that .577 were. That, if anything, would be the closest thing to a guaranteed one-shot-stop revolver the world had yet seen. If tearing a .45 hole is better than tearing a 9mm hole, then this should be the next step up.
Back to the .38/200, though, it can't have been anything to sneeze at. It's got the same momentum as a 100gn bullet moving at 1200fps, and if you subscribe to any sort of momentum equation for stopping power, it's not too far behind the 9mm. Plus the sectional density of that bullet is over 0.200, so penetration's going to be pretty good too. Might not do too badly on IPSC-style steel plate stages. The issue would be reloading time - the Webley bred the first speedloaders (straight-push Prideaux type), but they were for the reciprocating-action version (Webley-Fosbery) of the .455, and AFAIK never made it into the .38 version.
I've heard that the idea was that even if the lead didn't mushroom, the bullet was sufficiently long that it would destabilise inside its target and tear stuff up, much the same as the .303 MkVII bullet was deliberately weighted towards the rear with an aluminium tip filler.
I want one.