I'm no expert, but here's my opinion, FWIW.
- Head failures: I've seen these primarily listed on 2012 models. I've heard rumor this stuff can occur on other models. I haven't seen any documentation on it from searching, though. Are the 2012's that much of a concern?
- Head failures pt. 2: Has the aftermarket corrected these issues at all upon failure? I haven't found any explicit documentation on what exactly is failing. If it meant building the motor after the failure due to how it is failing, I'd almost rather pay for that than hope the OEM replacer works better next time.
Potential engine issues are the biggest factor in me requiring an ESC qualified car. The engine is McLaren-specific, so I would not expect a decent level of aftermarket support. Personally, I wouldn't expect the aftermarket to do a better design job than McLaren / Ricardo.
- Iris 2: Is it worth it? I'm considering narrowing my search to only vehicles that have had the upgrade but I don't know if it matters enough to care without experience.
It depends what you would use it for / how much it would annoy you. I class it as a 'nice to have'.
- Gearbox failures: I've seen some information on the seeping gearboxes triggering full blown replacements over what appears to be a seal. Has the aftermarket remedied this yet?
I've seen ones that have had just the seal replaced. If an ESC covers a whole new unit, I wouldn't complain.
For context, I don't mind a car out of warranty as I have my local Lotus garage who does a ton of work on anything from their Lotus to Astons and even the occasional De Tomaso, so they should be theoretically equipped to at least know when to say they can't handle it. Further, I'm 1.5hrs from McLaren Toronto (I'm on the States side by Buffalo). Last, I don't really mind doing some DIY - I've done enough work on the Lotus over the years plus other cars I've tracked, so this doesn't scare me too much. Acquisition of parts and aftermarket availability are bigger fears potentially just due to availability.
My view is that McLaren parts prices are very high, so I'd rather have the peace of mind of an ESC. It won't cover everything though. As above, my biggest concern would be the engine.
I have some skepticism due to other headaches that the McL forum seems to indicate that the factory warranty extension is even worth it due to their unwillingness to cover certain claims, but could be swayed into a warrantied car if the collective think here is that the cost benefit ratio is a near-guaranteed win. I don't mind paying for repairs out of pocket, but obviously there's no reason to throw away money.
I don't know if McLaren make money on the ESC. They obviously get discount parts and labour though, but will also want to protect their reliability reputation.