Really appreciate the detailed writeup, Fireball. You look to be the first to actually have done this. I suggest updating the original thread as well.
Please keep us in the loop as you gain more experience with the new brake bias. That will help others decide between the two available sizes. You mentioned tire width as a variable, but might also indicate your tire compound, as that will affect weight transfer to the front under braking.
Did you specify the red option and perhaps that caused a delay?
Are you going to tackle replacing the heavy sliders with a dedicated parking brake next? That has been done before, but isn't fully documented. Let me know if you'd like the incomplete info I have.
Based on just the ratio of piston sizes, the bias currently comes out to 53%/47%. But different tire contact patches and weight distribution will affect how it all plays out. Most of my weight savings on the car are at the rear (lighter muffler, lighter battery, and taking the passenger seat out), so besides my 205F 245R tire widths, my weight distribution may have shifted forward slightly from stock. The CP5316s would have yielded a 57%/43% bias (using the stock front caliper), which would have been a more conservative starting point, but I decided that I'd rather start with a little too much rear brake bias and then soften the rear bite with pads or a reduction valve rather than dummying-down the front braking. Also, the price for the CP5316 was over 60% higher
than the CP5317, so that made the initial choice even easier.
At Laguna, I was running on well-worn Bridgestone RE71R's, but I also sometimes run Hankook Z214s (C71 on the front and C51 on the rear). The "looseness" depends on how hard and late your brake, and how much trail braking you use, especially in off-camber or top-of-the-hill turns, so Laguna may not have been the most representative test case for general use. We'll have to wait for fall to try it on some other tracks. I should also add that I run on an OS Giken LSD, which has partial lock-up on decel (to combat "the cha-cha" under severe braking), so YMMV.
Because nothing was in stock, it didn't matter what color I wanted, so I ordered red just to be weird. From the time I ordered them in January, the planned lead time started at 8 weeks, and just kept moving out 8 weeks at a time. By May I was pretty much willing to take any color they had, but they just hadn't made any.
As far as parking brakes, I'm trying to be pragmatic (cheap). Eliseparts was selling a custom mechanical parking brake which was plug-and-play, but it wasn't cheap, and they stopped making them. If I really wanted to be weird about the unsprung weight of the stock rear calipers, I would just take them off and keep a block of wood in the trunk, as race cars don't need parking brakes. But my car's a street car, and I've developed well enough as a driver to know that my driving mistakes cost me a lot more time than an extra couple of pounds on each rear corner. Unless the track is really bumpy, or you regularly ride the curbs, I don't think the expense of custom parking brakes is worth the money. I'm all about "bang for the buck".
The lead times are just really long, in my experience. I've been going back and forth on using the lotus vs the 5317 vs 5316. BOE has the 5316 and lines available, but it appears to be the most expensive route.
I am a long-time customer of the Lotus shops here in the US, but sometimes it is just cheaper to source some of the standard items from UK shops, especially since the pound has been weak the last few years. There are also a ton more S2 Lotuses in the UK, so their customer base is just bigger. You just have to creep around on the internet with the right part numbers and specs, make contact with some of the Brits, develop some comfort with them, and see if their price plus overseas shipping saves you money or not. And unless the item is not in stock, it seldom takes longer to ship it here than if it was coming from somewhere in the US (but yes, the shipping will cost more). In my case I ended up purchasing the calipers from a "Tuner" shop in the UK because they were an AP dealer, and got the hoses from one of the UK Lotus shops. By purchasing the calipers from a domestic (British) shop, I was able to save a substantial amount of money, and ended-up getting a new set of 5317's for less than a lot of people pay for a used set of Lotus front calipers here in the States.
p.s. I need to test more, but I think that the 53%/47% brake bias may have actually gotten the car to the point where the ABS is now working properly
by modulating both the front and rear brakes. So far the only adverse difference I noticed was in trail braking in "unweighted" situations, which very well may be corrected by a lighter touch...