I don't get it. If a suite is so overly relevant for one particular stay - why not book one (using cash or point upgrades when available)? Anything else is a gamble. It may be a calculated gamble but still leaves you at the hotel's discretion. Remember, sometimes there are suites that never show up online so you won't even be able to stage a DYKWIA at check-in.
An SNA may increase your chances at an upgrade and will (if everything goes according to plan) give you some sort of suite confirmation before check-in. But that's it. It is not a guarantee but a request that the hotel may choose to honor.