How does this ultralight bikes (~800 grams) compare with the other carbon bikes like Pinarello, Focus?

I'm not going to compare top end bikes, other than to say I'd like to have one. Yes, its common to specify angles and tube lengths at that level. Those bikes are all low production volume, so there's a lot of hand building involved anyway.

If you ride a lot of miles, you already know that even a centimeter change in bike position can make a noticeable difference in comfort, aerodynamics and your ability to apply power to the road.

Even on mass produced bikes, a professional fitting (on a trainer with a power tap and heart rate monitor) can make a huge difference via changes in stem length, height, and saddle position.

BH is a well known Spanish bike company, not seen all that much in North America. Very doubtful that custom sizing is an option, companies making monocoque carbon frames don't create custom molds for individual riders, much too expensive. It is possible to customize if lugged carbon construction is used. Ultra lightweight frame are often very stiff and uncomfortable, if you are interested in this bike you should test ride to see if it suits you