I have a Clapp 9 mm and a .45. In both instances I changed the brass bead front sight for F/O ones from Novak. The gun is a pleasure to shoot and the Singleton checkering is terrific. Both versions seem tighter than normal production, although they are supposed to be production guns. I use a bushing tool for field stripping on both, not needed on my Gold Cup. And accuracy is better than I am.
The 147 grain is usually considered match ammunition. It's also subsonic which is good for suppressed pistols. 115 grain is or was the choice of law enforcement for years...the Federal 9BP was considered the gold standard of 9mm defense ammunition but I'm sure it's been bypassed in technology by now. 124 grain is NATO standard.
As already said...try an assortment and see what the pistol likes best.
I went looking for the 9mm Wiley Clapp too, planning on sending the slide to Novak for different sights. Dealer 30 miles away can't get it from their supplier. Found another possible source, drove an hour to get there. Fellow there asked what I was looking for; a Wiley Clapp 9mm I tell him. That's when he started making facetious comments about The Clap'. Then tells me never heard of it and I would have to look on internet to transfer it thru him. Hmmmm okay, thanks. Another hour and I'm back home searching the net again. Found a Colt Commander 9MM, O4942XE all steel, decked out with the type of sights I prefer and other features similar to the Wiley Clapp. Ordered it for transfer thru closest dealer, CBG. Very nice in my opinion, only Series 80 Colt I have. Fired 115, 124, & 147 subsonic with no problems what so ever. Quite accurate with all loads I tried. Let another fellow it at the range give it a try; now he wants one too.
Kinda sorta still looking for the Wiley Clapp though......