But the gt86 is much lighter.
This is listed at 1395-1470kg vs the new BRZ being listed 1275-1305 - it's significant but not a massive difference - don't forget, most people aren't as singularly obsessed with low weight as you are.
It's a different car with a slightly different market anyway. It's a higher specification of car for anyone who cares beyond outright performance statistics - the Supra comes with plenty of creature comforts and tech and toys that the GR86 won't. It'll also be more readily improved with simple software - if VWs 230/245bhp 2.0s can map to 300+ easily, i'm sure this engine will too, with a considerable torque advantage too.
Not sure How I feel with the 2.0 version being priced at £45k when compared to the new BMW Z4 2.0 model priced at £38.8k
...which follows into this bit slightly - the 2.0 Supra at £45k, has a few things standard that the cheaper Z4 needs optioned, sometimes as part of expensive packs, such as adaptive suspension. I don't know without trying to make a like for like car but I reckon if you configured a Z4, M Sport trim and tried to add the options the Supra has, it'll be near enough the same price.
Edit - as for £45k being too much, consider that this is meant to be competing with the 2.0 Cayman and 2.0 F-Type, which start at £45k and £56k respectively and it doesn't look quite so mental - despite being down on power on paper by 40bhp, it doesn't look to be much slower than those cars. I'm sure at £45k the Porsche isn't coming with Adaptive Suspension, Sports diff etc. either.