Hello Andy,
Well it's absolutely not tricky at the limit, the car telegraphs it's intentions very early on so you can catch it fairly easily. It's more the fact that in the conditions I drove (Michelin Sport cups, 7-8°C) I found the limit rather low compared to other cars which tend to push the envelope as far as it goes and when it comes it's difficult to catch. In that sense the GT3 is more tricky because it will be more nervous, so like I said you can't let your concentration down. But I like that I feels more "real" less artificial. And it seems it's easier to hooligan the car at lower speeds.
Concerning the electronics, well they do cut in very late even in normal mode, the car will allow the back end to step out a good metre before intervening so even in normal mode it's nothing for beginners but it allows you to play without spoiling the party too early. But you have to be aware of it.
Comparing to the 458, well it's a bit the same thing actually, with far less drama than the italian. But both can have the surgical precision ideal line carver feeling as well as being able to "Ken Block" I would say the GT3 has a tighter, more "neat" feel to it.
Like I said before I find it feels more like a pre 993 911. Overengineered but still giving you that racecar chattiness.
I've only driven de McLaren for a short time on track and found it totally uninvolving, very efficient (I doubt there's a lot of faster cars you can buy) but no fun, no nasty tricks. It only feels overengineered.
To sum up the GT3 has a nice "analogue" feel to it you don't really feel what everything is doing in the background, the Gearbox is the best of those I've driven yet (ferrari is smoother in full auto), when you're pushing it feels like a race sequential would and the car pops and bangs, it's quite impressive and bagloads of fun!
I'll keep you posted how it is with adapted tires.