So, after a couple of months of being inactive on this, I've decided to try to get Yosemite on T450s again. I tried a fresh install, and now, HD5500 drivers work. The problem with my previous attempt on getting them to work was that I had mistakenly used config.plist from the EFI partition (which I then modified but somehow forgot to add the patching for Intel graphics drivers) and patched the kext myself with a binary editor. This time I copied the config.plist (attached to guide for CarbonX1 2015) and added "-v". After that, I continued patching DSDTs exactly as per the mentioned guide.
So, for now I can report that the system functions somewhat correctly, with working audio, bluetooth, USB 3.0, graphics, trackpad&keyboard, camera, microphone; basically all essential functions/components. Wifi does not work, but that was expected, and the battery status icon does not show correct percentage (also, it does not know whether the power adapter is connected, it always shows as though it's charging).
I have, however, noticed that the operating system can be really slow sometimes in basic operations - the behaviour I am experiencing is that it hangs for a few seconds (or sometimes even for half a minute) after I perform a certain operation. It is really annoying. So I checked the Console for errors that occur when the system gives me the beachball and I found out that there is a diagnostic message that keeps repeating.
Code:
09/09/15 14:16:15,202 powerd[48]: SCPreferences(Power Management configd plugin:com.apple.PowerManagement.xml) lock held for 1.643 seconds
com.apple.message.domain: com.apple.SystemConfiguration.SCPreferencesUnlock
com.apple.message.signature: Power Management configd plugin
com.apple.message.signature2: com.apple.PowerManagement.xml
com.apple.message.value: 1.643
SenderMachUUID: 3533AF53-906D-38D6-B29A-DDDEB6FD91CD
I've started to think that the battery status icon is somehow connected to the occurrences of system idleness. One thing more that I have to point out here is that the T450s has 2 batteries - 1 internal and 1 removable. And as noted, I patched the DSDT according to the guide for CarbonX1 (so i used the ThinkPad X220 battery patch). I'm guessing that this inappropriate patching might be the underlying cause for power management errors.
I'd be glad to hear your advice on solving this issue.