To be honest Dan I feel the standard GTA set-up handles bumpy roads at speed better than the FSD with Eibach Pro-Sport springs, even the Mrs thinks so
I'm a big fan of the standard GTA setup and agree with you (assuming its not knackered), the softer the spring the better it will be at absorbing roughness from surfaces, thats pretty much a fundamental rule of suspension. But replacing it when its worn out is more expensive than a set of KW coilovers so not many people ever get to experience how good new original GTA suspension is compared to new Konis/Bilsteins/KWs. Most peoples' experience of GTA suspension is old worn out stuff, they only start to think about it when it doesn't feel right and then they replace it with something aftermarket and are amazed at the night & day comparison. You & me are in pretty unusual positions of running multiple cars at the same time with different setups, it does open your eyes a bit.
I've got FSDs on my 147 GTA (not the track slag) at the moment with standard springs and its bloody lovely to drive around the Cumbrian lanes on. I'm never carrying enough speed to really need stiffer springs except for on the odd big roundabout and the suspension absorbs big and little bumps easily as well as my Mrs's Mk5 Golf GT which is high praise indeed. In addition to that the car is amazingly adjustable and communicates exactly what the tyres are doing, you can feel them breaking away and adjust your line with the throttle as much as with the steering wheel. It handles/rides feels a bit like a grown up Ford Puma. I can't see how the car would be improved by fitting Eibach Pro springs except visually. Even the track slag is slower on these roads as its too low and too stiff, despite being on £1500's worth of Intrax HDAs.
Suspension choice is as much about where and how you're intending to drive the car than anything else. My 147 GTA would be fairly useless on track with original springs and rollbars compared to a lot on here, but having had KW coilovers on previous 156s (V1s & V2s), I'd not be able to maintain the same pace across the fell lanes if I had them than I do now. FSDs are definitely better than OEM GTA shocks but take the spring choice out of it; you think FSDs give bad ride quality with Pro springs - I've had the unfortunate drive of a GTA with Pro springs on original shocks - over 100k too and it felt like it was going to shake apart! FSDs & Eibach Pro springs were a great all rounder for me when I lived in Bedfordshire, I've said before a million times its a great all-round setup if you want your car a little more focussed than how it left the factory. As they say there is no such thing as a free lunch.
I have doubts that the FSD's work with lowered springs, I feel there isn't enough suspension travel for them to work. I want to change the suspension on the GTA to cure the lack of rebound damping but don't want to drop the ride height much if anything at all and I'm confused as to the way to go. The popular route seems to be coilovers, with KW the #1 choice. The top arm and ARB bushes need changing and I'd like (Loz) to do the work all together!!
I have no concerns with FSDs on Eibach Pro springs, they're not all that much lower than original GTA springs. Less than 20mm. On H&R or Eibach Sportlines I think you'd be right. As for KW V1s, I had them on my 10v, I didn't want to drop it too low as those cars have a very deep sump which is prone to taking a whack; the KWs were great for hammering around on but I found it quite unsettling on the motorway, it didn't take big bumps well. I had the adjuster set near the top, to give maximum strut length (and thus maximum ride height) but this meant the spring was quite heavily compressed even before the weight of the car was applied. It rode better when the adjuster was set towards the middle but it was too low for me then and I did scrape the sump. What would have been better is setting the adjuster at the middle but then using a longer and softer spring to give back some of that ride quality. This is exactly what Ned has spec'd with the Eibach Pro Street S coilover kits that we sell as opposed to what you'd get if you ordered it off a normal retailer. Bare in mind the Eibach coilover bodies are rebranded KW V1, how they feel is much the same except with better ride at a higher ride height setting. Although these wouldn't be suitable for going as low as the KWs.