Good questions. I haven't played around with using synthetics to get better fills. I think this is primarily because with exception of a few longer dated flies in commodities I'm 99% trading one day plays and the bid / ask spreads are the usual tightness. Early exercise is a potential risk but one that is very minimalized as all these flies are put on with a directional bias with the short strikes well out of the money and then once again being held for only a day. If I stay away from earnings which can cause large after market moves the risk is pretty low. For example is I put on a 10:20:10 call flay in TWTR 50-60-70 and the underlying is at 51 no one would exercise the 60 calls. I guess someone could but it would be an error on their side unless they know something no one else does as it's $9 out of the money with one day to go. But I do worry about how it would effect the structure of the fly that is left if it ends up being 10:19:10 the next morning. The margin isn't too much of a risk to me as I'm trading with only 3-4% of my capital exposed each week for the one day period but on a large priced underlying it could present an issue. I could always just leg out of the options and I do do this sometimes for more profits. In the above case just buy the short guts back on the open and buy 100 shares of stock and then I'm left with 10 of the 50 calls and 10 of the 70 calls to work out of. If I were holding flies for a couple of weeks I think it is a much more valid risk.I also remember a user on the journal who suggested synthetic setup for the short straddle part of the fly, to save a few cents on the fills. Have you considered it?
I also think it might be good to cover eventual early excercise, I use IB and their margins are funny. I can see that in the journal you never had to deal with early assignment but it is one of my worries when trading multiple legs or positions.