The easiest way to do a what-if is to look at the mid of the spread right now and the currently trading ATM strikes of the same spread (just shifted down or up).
If SPX goes down right after I bought the spread, the loss could be higher because the premium on both puts are much higher early on the day.
Right. You'd have to have a tighter trigger level for closing the trade initially. Really this isn't much different than just taking a directional trade like ES traders do. You need to be right pretty much immediately or else you're looking to get out.
How do you plan to trade the stop? Is it automated? Because when there's a rapid move, you might not be able to close it in time. If it's automated... what if the b/a-spread of the put widens? And, what about after hours trading? Gaps?