I do like the stuff Derek Brown plays, also an easy to watch tutorial! I can get the first few overtones off Bb, B, C, C# and can sometimes slur up to D and overtone A, but when it comes to Altissimo that's another story, I cant hit anything, but I will persevereIt's not just about fingerings, it's about embouchure - how much lip pressure you use, the shape and position of your tongue, shape of your mouth, how much air pressure you use...
Try to shape your mouth like you're trying to sing the note.
Practice overtones off the low G.
Try these mouthpiece exercises - Saxophone Frequently Asked Questions
and read this as well, it may be helpful - Saxophone Frequently Asked Questions
more advice can be found on the internet, some of which may help and some won't. I didn't start to understand Sigurd Rascher's Top Tones For The Saxophone until after I learnt how to play in the altissimo register.
In essence, you're deliberately trying to make the reed squeak in a very controlled way, so it's contrary to a lot of what you've been doing so far. Some people suggest altissimo is easier on a harder reed, but I didn't find that to be the case and a 2 1/2 or 3 should be fine.
Some mouthpieces may be hard to play in the altissimo register - but Sigurd Rascher must've been playing in the altissmo using the kind of low baffled vintage pieces that are beyond my capabilities
here's more food for thought -
Playing in the Altissimo Register - JodyJazz
12 Tips for Hitting that Altissimo G Like a Pro » Best. Saxophone. Website. Ever.
Approaching the Altissimo with Lenny Pickett
In the interest of full disclosure, I also have a David Hite (my first upgrade mouthpiece recommended by my teacher). I forgot about that one. Also, I just ordered a Jody Jazz Jet number 7. It arrives Tuesday. Sadly, I am not immune to GAS. Sadly? Who am I kidding? I love it!The other one is a Beechler but an M6, medium chamber, smaller tip opening. Those are the ones I play. I also have a Selmer S80, a Yamaha 4C that came with my YAS-475 and a no name plastic one that cane with my old student alto. But I never play those.