I really struggled to notice the difference in ride/handling when the rear of my GTV was powerflexed. I think you have to be a track day specialist to really make a comparison. I fitted them really just because it is the fit and forget solution to the crumbly Alfa originals, so I would say don't worry. The strut brace seems to be the best improvement for Spiders especially.
If it's not too late I would strongly suggest you don't bother powerflexing the front. The bushes and balljoints are integral to the wishbones and all seem to have the same lifespan - 26k in my experience. Unless you know differently, the balljoints cannot be fitted separately so you will still have to replace the whole wishbones - containing bushes - with the same regularity, even though the poly bushes will be fine. So this mod seems to be only relevent for competition driving.