Hello viersen,
I just finished installing a Samsung 850 Pro (512GB) on my mid 2010 MacBook Pro. My MBP has the same specs as yours: Model 6.2 with Intel Core i7 (2.66GHz), 8GB of RAM, and the dedicated NVIDIA GeForce GT 330M (512MB RAM). I also bought mine with the then-high-resolution-display of 1680x1050. Anyway, the SSD works great! It was actually quite easy to do the swap. I backed up my Mac on an external drive using Time Machine, I booted up the Mac with the new drive installed (using a boot up flash drive), formatted the new drive, installed Mavericks, and restored all my stuff from the Time Machine backup. The machine is considerably faster. Nevertheless, you are right. The mid 2010 MBP has a SATA II port of 3Gb/s that can only support a bandwidth throughput of up to 300MB/s. That said, instead of getting read/write speeds of up to 500MB/s (which the Samsung 850 Pro supports) you only get the maximum SATA II supports. It seems that my read/write speeds are consistently in the vicinity of 250 to 270MB/s (calculated using free "Blackmagic Disk Speed Test" app from the Mac App Store).
Here are my considerations. With the original drive (HDD 7200rpm) I was getting read/write speeds between 70 to 80MB/s... now I am getting over 3x that! So, even though the SATA II doesn't allow the Mac to use all the speed the SSD provides, you can technically say my Mac is 3x faster! And you can definitely tell it is 3x faster! So, the questions are:
1) Does the 850 Pro work on the mid 2010 MBP despite the limitations of its SATA II port?
Yes. SATA III is backwards compatible to SATA and SATA II. Nevertheless, when the SATA III (6Gb/s) is connected to a SATA II (3Gb/s) port, the drive can only reach up half the sequential read/write speed rates because SATA II only supports half of SATA III bandwidth throughput speeds.
2) Is it worth it to pay (as of today) $50 extra for the 850 Pro instead of saving those $50 and get the 840 Pro?
I think is worth paying the $50 extra and get the newest drive with the newest technology. The 840 Pro has 5 year limited warranty. The 850 Pro has a 10 year limited warranty. And although you probably won't have your mid 2010 MBP for that long, I'd rather have a drive that is rated for a longer life span. Besides, both drives (840 and 850) are SATA III.
I'd say, go for it! Get the Samsung 850 Pro! It works great!