Correct. You can have OtherOS++ on 3.55 anyway (altho I'm not 100% sure it works with Cobra, I will test it myself once I receive my dongle).
About the heat building up issue, it's not something you can't fix. The issue is with thermal paste. The thermal paste tends to dry up after 2-3 years of use. Most people replace the thermal paste between the heatsink and the IHS, and notice that it doesn't help much. The reason is simple : there is also thermal paste that dried between the IHS and the die. What you have to do is remove the IHS's (which is a delicate operation but not that hard), change the thermal paste under it, stick the IHS back. I suggest using a non-conductive paste (AS5 is conductive so avoid that) like Arctic MX-4.
That said, I doubt even if doing that your console will not YLOD, but it will decrease the chances a lot. I bought a CECHA01 used model off ebay recently, and its fan would get really loud after 10 mins. I took off the IHS's, and changed the thermal paste (I'm waiting to receive my MX-4, so I used AS5 for a temporary test) and guess what, after leaving it on for 30 mins, the fan was still more or less silent.
PS3 Slim CECH-3004A 160GB
PS2 Slim SCPH-70004 - FMCB 1.8b