The usual upgrade path from W7 Pro to W7 Ultimate is via an "anytime upgrade", for two reasons: it does not require a fresh install and it is normally cheaper than a full or upgrade license. So if you bought an "anytime upgrade" then you CANNOT simply install Ultimate and type in that key; if you wish for a fresh install then you must reinstall Pro with your Pro key and then go through the upgrade process with the anytime upgrade key.
SLP or NONSLP has to do with the W7 install software; it has to do with whether you have to type in the product key or whether it is embedded in the computers' BIOS and pre-activated. If your computer came with Pro and you want to install a full or upgrade version of Ultimate then the install will definitely not activate via SLP because, even if your computer has a SLIC table, it is set for W7 PRO software activation. You will need to type in a valid Ultimate product key if you install Ultimate.
If the product key you type in is an OEM key, then when you type it in, your W7 Ultimate will become an OEM version. This has more to do with legally transferring the software to another machine later than it does with the functionality of the software on this particular computer. An OEM license locks it to this particular computer and cannot legally be transferred.
It is my understanding, SLP or NONSLP, OEM or nonOEM, the software is the same, it is all about how you have to activate it and what you can legally do with it once it has been installed and activated on a machine already.