I recently bought two IWC deployment clasps -- one for my Spitfire Mark XVI and one for my older Spitfire Chrono. They weren't inexpensive, but they are very nicely made. On watches of the class of IWCs I also prefer OEM straps, etc., to aftermarket furniture.
The original straps work reasonably well for me with the short end at 6 o'clock. I'm pleased overall, although the deployment clasp would probably work even better if the short end were 5mm to 10mm shorter. But when I was ordering the clasps I discussed the matter with my AD, and it didn't appear that deployment specific straps were available from IWC for those Spitfires.
I vacillate between preferring deployments or pin buckles. I also agree with Paul regarding the merits of the pin buckle. And even though the deployment clasp probably is a little easier on straps, I do expect to have to replace straps due to wear anyway.