Hi Daniel,
Welcome to the Community,
1.Replacing HDD with SSD does not void the system warranty,however the new hardware(SSD drive) will not be covered under the system warranty.
2.The Inspiron 7520/15R (SE) systems are offered with 2.5" SATA 2 Hard Disk Drives.Any 2.5" SATA2 or SATA 3 SSD would be compatible.
3.Yes,you are correct and even if you buy a SSD which has the faster transfer rate it will function with reduced or limited speed which SATA 2 port offers.
And also these systems are offered with SATA 2 and 3 mSATA Solid State Drives. These systems support the combination of mSATA Solid State Drives with a SATA Hard Drive which enables the use of several technologies found in Intel® Rapid Storage Technology:
Intel® Smart Response Technology: This technology utilizes the speed advantage of SSD in caching where the frequently used information is stored on the SSD instead of the Hard drive.
Intel® Rapid Start Technology: This technology enables the system to boot back with-in seconds after getting into hibernation.
Note:If you wish to have mSATA preinstalled on the system, you will need to contact your regional sales representative and configure the system accordingly at the time of purchase.
Thank You