I can confirm that if you polish it, the strut brace is indeed very shiny and probably adds 10 bhp despite the increase in weight.🙂
The short serious summary answer is, I don't think there's any significant handling advantage in having a front strut brace for normal or even fairly spirited driving. On the track, on the limit or in a smash where it should add a bit more rigidity perhaps it's worth having. To improve the handling without compromising the ride, fit good tyres, coilovers and better ARBs.
When I first fitted it I would say I was 'in tune' with the car and was convinced the front top strut brace would make no difference at all but in practice I felt it did make it feel a fraction more stable. It's a shame the old forum notes are gone as I remember doing a very brief review.
The SC is good fun and more capable than most realise considering it's a flexible cabriolet but If you want the car to handle and go fast the first thing to do is to leave the roof on, the windows up and think about fitting a roll cage. They are strong, well built cars but you really don't want to be upside down on the tarmac or in a ditch in one of these.
Coilovers, wide wheels, wide sticky, tyres, uprated front anti-roll bar (Aristo/GS TTE) and an ISF rear anti-roll bar all definitely make a noticeable difference. Adding the underbody brace also stiffened it. Having a proper leather steering wheel to grip helps too.
Mine isn't a posh Cusco brace but it has the optional authentic Lexus sticker to impress Uncle Bob the race scrutineer.