An update on this, for those who may be thinking of doing something similar - it's all done, and seems fine.
2009 Lexus GS300 SE - I replaced the original Lexus adjustable (normal/sport) shocks for after-market non-adjustable ones, without problems.
I bought KYB shocks (KYBS51108) at roughly £140 each including VAT (Lexus originals were £360 each).
A set of two SUPLEX coil springs (35336) (OEM-Number TOYOTA 48231-30A50) for £111 including courier costs from Germany via UPS.
The absorber control actuators (part 89241-30040) were simply removed, and the car feels as though it has reverted to the "Soft" or "Normal" suspension setting. No error messages flash up on the dash, and the sport/normal switch still lights up the notification on the dash as before - I presume this switch still continues to tighten up the steering as well, it certainly feels as if it does, although the difference was never that marked, I thought.
The actuators seem quite rare, so I'll hang on to them and maybe Ebay them at some stage.
Car drives beautifully. Job done. Thanks to those offering suggestions.