I did the 562XP spike thing on my 550XP. Make sure you use the right dremel bit (for hard metals) to elongate the mount holes, and it will hold just fine. I still would never use the 550 for felling - it is a great limbing and bucking saw, but just not up to the task of felling.
As a noob and a firewood only cutter, I gotta add my 2 cents - I bought a 261C a few of months ago. I only have about 4 tanks through it. It came with a 20" bar with RM chain. I put double dogs ( for a 362 -thanks Stihlman
) and I have used it to fell quite a few trees. Comparing the 261 and 550 side by side, if you're determined to use a 50cc saw to fell, the 261 is by far the better choice. I absolutely love the 261 -that saw just pulls like crazy and feels stronger than the 550. The standard bar on the Stihl is also much higher quality than that on the 550.