Well, KitKat 4.4.4 Release + latest CM11 kernel from LegacyXperia are quite stable and usable on my Xperia Ray (ST18i) in my usage scenario. However, I have most PAC features turned off (considering very low RAM available, and not so powerful CPU), and most app's data and Dalvik cache moved to external F2FS data partition on SD card. Chrome is almost unusable on this device (too slow, blank pages are often), so I reverted to standard browser. Youtube is hardly usable. Skype is slow in response, but okay in performance, including video - using it for work. Gapps are another point to consider, you can update it independently, but from compatible distribution. To sum it up, files to flash are (for ST18i, look for similar for yours one):
pac_urushi-nightly-20150121.zip (MD5: bdabcc7f5f4087312a03f73eb8ff37ba)
cm-11-20150511-UNOFFICIAL-LegacyXperia-urushi.zip (MD5: 348241e78b729ae80e858ba2044d6176)
gapps-kk-20150222-minimal-edition-gps034-240dpi-signed.zip (MD5: 36132a531d23f4b218fd0176ea9924a7)