For what it’s worth, I have the Rapide CLXs, only got them last month as an upgrade for the S Works SL6, which I usually ride on Mavic Cosmic SL 40 (something like that forget the exact name).
Funnily enough I went from last gen S5, to the S Works SL6. Same journey you’re considering but older gen.
From my limited experience - the Rapides genuinely do punch above their weight. They’re bizarrely stable. I don’t get this part, it still pains me to understand how a large surface like a deep front wheel can be stable in a crosswind. Before I bought them I had riders telling me the same thing, but I figured it was mostly hot air.
I bought them due to their depth v weight, think they’re best on market (apart from tubulars), but the stability really is legit. I ride in coastal windy conditions and country lanes with heavy gusts breaking through gates on downhills etc, it can be properly nasty out there, but these things take the drama out of it.
In terms of out and out speed in a straight line, they’re like a Zipp 404 I’d say, which is one of my fav all time straight line speed wheel sets (ran them in tubular though). But the 404s are horrible in winds compared to the Rapide CLXs. Totally different league. 404s might still be a sec or two faster in a straight line TT (maybe not, I don’t know), but over a windy rolling course with fast sections the Rapides will be much faster. I can bomb down stretches on these things that I definitely couldn’t on 404s. Weight wise the Rapide CLX’s are much lighter than current 404 clinchers, maybe 200 to 300g off the top of my head. Big deal now when you’re running disc brakes - so much harder getting a lighter bike on deep sections.
Absolute murder trying to get clinchers off the Rapides though, be warned! Only tried Corsa G2+ clinchers so far, but they are not puncture friendly. Praying they stretch before next puncture and they get easier. Even getting the bead into the channel needed brute strength. Can’t see it happening in a bad day with frozen fingers.
Wouldn’t be fair for me to compare old S5 to SL6. I went from rim brakes to disc brakes, and tubulars for fast wheels to clinchers again. Weight wise even the Tarmac is supposed to be a climbers bike or all rounder, I haven’t got the weight down to the S5.
SL6 is so much nicer to climb on out of the saddle and it corners like nothing I’ve been on before. Sometimes I wonder if it could u-turn at 50mph. It is a touch twitchy. I’m well used to it by now, but it’s always there. Only regret is not sizing down on the Tarmac like I did with the S5. 178cm on a 56cm SL6. Feels a bit normish, but at least my feet don’t hit the front wheel now.
Huge Cervelo fan, took a month on the SL6 but it turned me to the dark side. Would be very hard for me to move off from the S Works brand now. The Specialized customer service in the UK is ridiculously good too. I have had issues, and Spesh were on it each time (cracked Aerofly II bars, seat spot slipping etc - new parts out and no nonsense either).
Wall of text but might be something in there that helps. I’d love a go on your S5D by the way. Most exotic race bike on the market and I know a rider who had my version and yours and he says yours is a huge step up in every way. He’s also moved onto the SL7 recently though.