although 0-60 figures does not paint the whole picture when it comes to an overall performace of a vehicle it surely does sell cars but saying that, it depends on the level of performance you looking for.
the performace figure of the alfa compared to the 300h is about the same in terms of 0-60 but i will put my money on the 300h when it comes to rolling acceleration. since the batteries are capable of generating 300NM of instant tourque when fully charged thats equivalent to a 6 cylinder 3.0 litre NA engine so for rolling acceleration up to a point the 300h will feel quicker
and because Lexus tends to have smooth drive trains you dont actually feel you are going quick but your are. my 300 sits at 70mph on motorway and it feels like am doing half that. side to side the Alfa spider driver will feel like they are going fast at 70mph and the 300h driver will feel like they are doing half that.
but for out and out performance hybrids i would say i would say look at 2-3 year old GS450h's or the Infiniti M35h's and pocket the change on splashing out on a new 15 plate £30-40,000 IS300h.. since they have done most of their depreciation and can accelerate to 60 in less than 6 secs infact the Infinit hybrid holds the world record for the fastest accelerating production hybrid neither of the German hybrids can match it. IS-F might be over kill in terms of what you looking for?
thats a 450h being put through some stress and it handles it without breaking a sweat