All our systems have price as a filter and the software reads the average of several bookie SP prices 30 secs before the official close. Therefore all Lay bets are placed SP.
Lay SP is definitely weight of money sensitive. At $2k liability during the week, I have never had an order not filled on Gallops. I have posted previously Betfair statements where I have definitely moved the SP outside of the last traded Lay price big time. I have only had one SP fill shortchanged but from memory it was a $10k + liability bet on a longshot. I did post it on a Schmile thread, but can't find it.