In an effort to keep my X230 snappy for a few more years until I find/make a newer laptop to my liking, I finally caved in and bought an i7-3612QE board. Posting some observations and thoughts based on the questions I had prior to buying. Previous CPU was the i5-3320M.

Setup

• Debian 12
• XFCE
• 16GB DDR3L
• Two SSDs
• Hyper-threading disabled
• 1vyrain BIOS with classic keyboard EC patch

Performance

• Not literally twice as fast, but the improvement is quite noticeable
• CPU no longer seems to struggle while loading Javascript-laden websites
• Rarely hits 100% CPU usage, even on Youtube (sadly bloated enough to be a sort of benchmark)
• Single-core tasks are only slightly better than before

Thermals

• High 40s at idle to mid 60s when busy
• Feels cooler compared to previous CPU, which I assume is due to the CPU usage being lower across all tasks.
• Did not upgrade to the AVC cooler. Toshiba cooler works well and is quiet as ever.

Battery

• About 4.5 hours of office tasks and light browsing from full to empty with an aftermarket 55 Wh 6-cell at 98% health
• Wattage in the mid 9's at idle with brightness at a comfortable level for a well-lit room. Increase from low 8's with the i5-3320M.
• When doing actual work, wattage hovers from 11 to 15 watts
• 16 to 18 watts watching Youtube videos
• SLT1 IPS display does consume ~1 W more than TN. I installed tlp but left it on default settings.
• 65W Lenovo charger only works when in sleep mode or shut down. In normal use, however, it will not draw the full 65 W. A 90 W charger or a 65 W GaN charger that the X230 believes is 90 W will both work (my 65 W GaN charger worked well and did not overheat, YMMV)

Other

• Make sure to enlarge the cutout on the black sticker on the underside of the cooler since the 3612QE die is larger
• EC flashing will require a 90 W charger or a 65 W GaN that the X230 believes is 90 W.
• Factory CPUs have BGA package underfill. I have not checked for myself, but it is likely that the upgraded CPU does not have underfill. This should not affect day-to-day use, but the lack of underfill will make the BGA solder joints more susceptible to fall and vibrational damage. Liquid may get trapped underneath in the event of a spill.

Value

• With shipping and taxes, the upgrade costs about 200 USD and takes 2 hours. The total cost-to-date on my X230 built from parts is around 500 USD.
• Do not think of this upgrade in terms of how much performance you get for the price. Think of it like upgrading and daily-driving a classic car. If it brings you joy to daily drive an X230 as it does for me, then it may be worth it.