Got this T540p for free from work, it came with a dual-core i5-4200M, 8GB of RAM, the awful 1080p TN panel and the GT730M dGPU.
Upgrades: New TrackPad (T450 Synaptics), i7-4700QM, 16 GB DDR3L RAM, AUO B156HAN01.2, and a good clean (yep, that's an upgrade in my books).
The i7 runs quite well in there, I had some doubts about the thermal situation since it's a 47-watt chip in a dGPU model but so far it holds up pretty well with a -70mw undervolt and Arctic MX-4 thermal paste. Idle it gets up to around 45°C, under load I've never seen it get hotter than 75°C. In addition to Throttlestop, I'm using TPFC on the "smart" setting to keep the noisy Toshiba fan from ramping up and down every .2 seconds.
The GT730M isn't good for much these days but it runs older games quite well. All in all, I ended up really liking this ThinkPad. It's very annoying to take apart, though.
Lindows.. oh windows