So, what works on one engine will work on the same engine in another car.
This is exactly the fallacy I walked into the performance world with years ago and was quickly squashed by evidence. In the late 90s and early 2000s, it was extremely popular to buy a handheld tuner that would come with some changes it could make to your car. While it's possible this era of "off the shelf" tunes were less scrupulous than the likes of BOE, I have no evidence to assume it so. The result during that era was that some people would see a few hp gain, some would see no gain at all, and in some rare cases they'd see a loss. The tuning community across the board learned that these are useless compared to the gains you make while making small changes a bit at a time to the car in question until you find the optimal spark/fueling map for the car in question. While it's counterintuitive that this would all be true (Hey, these cars rolled off the assembly line together right?) and while I've encountered various explanations for why it's true (cars at different altitude, different gas, motor components in different degrees of degradation, tuner is more conservative when producing a tune for the masses rather than tuning a specific car, etc) none of that matters in the face of cold hard evidence: it's been observed that there's more to be had with a custom tune, and that experiment has been repeated countless time. It's very likely
that all cars will see very similar
results with a BOE off-the-shelf tune, but unfortunately it is not guaranteed
that all cars see the same
If my years of experience learning this lesson for some reason doesn't apply to the Elise, so be it. I'll be surprised, but so be it. Perhaps BOE and others have figured something out that "diablo" and all the other "off the shelf" tunes from back in the day couldn't. Either way, I'd only be proven wrong by performing a custom tune and then applying their tune and comparing, and that experiment is prohibitively expensive, so I'm going with the evidence from the rest of the performance community and am stuck in a position of skepticism. If the BOE tune does reliably provide something, I'm not certain they've left 0 hp on the table.
2. This upgrade is not based on "blindly make(ing) fueling and spark changes". It is well engineered and thoroughly tested (by BOE and many LT members).
You'll have to forgive me, I used my own terminology here which was ambiguous and you've interpreted differently than I intended. Any tune in which you don't check for the results on the car it's applied and continue making adjustments based on that feedback is one which I'm describing as "blind changes" since we don't have a standard term for this type of tune. I feel that "blind" is a good way to describe applying a tune without testing it and adjusting it, no matter how thoroughly it was tested on an identical car. I assume that these providers, such as BOE, are doing the best job possible, thoroughly testing, etc. By my definition, BOE and all other tunes available are what I would call a "blind tune" and I don't mean to imply any lack of diligence or capability on the part of the provider. There's only so much you can do.
5. You will not see a 20 hp gain.
I was afraid you might misinterpret that. I have no such naivety, I'll restate for clarity: I am disappointed in my car's performance by about 20whp, and while I would never dream of getting that out of a tune, I'd be very happy to learn that there is some guaranteed gain to be had out of a tune, even ~5whp.
The gain, as Lostpawn points out, is where we used to lose 20 hp right before cam switch, and the fact that that switch comes on at lower rpms.
Yeah, the lower change over seems like a no brainer and I'm planning on that no matter what. That's ostensibly a single constant change in the ECU that anybody could do and couldn't possibly screw up. It can be had from anywhere, including the "Sport ECU" that I believe Lotus offered themselves. I'm more interested in the gains from spark/fueling across the rest of the powerband. That's the tricky bit.
Anyhow, dave answered my question, so that's much appreciated.