2500 P Lizardmen, allcomers

You raise some interesting points. It is certainly true that the first turn, your magic is generally weaker than Wandering Slann, as you do have to swap out some spells. I have however found that something like a Hand of Glory (great on BS if you cast it on a Stegadon for the Bolt Thrower), a magic missile, and a Walk between Worlds is often easily cast, and then swapped out for something better. There are however, things that can go wrong. Rolling double 1s is terrible (even though the channeling combo mitigates this somewhat), and if you lose concentration it is also pretty bad. If you do get one spell cast, you can get what you need, and you are set for the rest of the game. Even better however, is that your magic will increase in power for the entirety of the game. If your opponent has that lucky crash through with his Demigryphs that you expected your Sauri to hold: get searing doom. Infantry getting you down, try for AOE's, etc. So, especially in the first turn, i would agree that having signature slann is better. 

Due to how magic works in 8th, i think getting better and better spells has compounding advantages. Let's say there are 3 spells you have right now that are effective, and you have enough power dice to cast all of them. With signatures, that might be something like searing doom, fireball and wyssans. Power level of the spells might be an 8 out of 10, and two 6 out of 10s. Your opponent probably knows this, and dispells the 8 out of 10. You end up with 2 sixes. Now imagine that due to spell changing, you have those 3 spells all up to 8 out of 10s. For example, searing doom, walk between worlds and fiery convocation. Once again, your opponent gets to dispell one, but this time, leaving you with 2 8 out of 10s. Note that the opponent dispelled the same amount of spells, but that your final value of cast spells is actually higher than expected, because your opponent only gets to deny the best spell. 

Another upside is the unique nature of high magic. I disagree in discarding Drain magic soon, Drain magic is fantastic in the sense that it does something no spell in the BRB can do: retroactively kill hexes/buffs. It sure looks flashy when a purple sun kills an army, but more games are decided i would say by a good Wyssans at the right time than a purple sun. Drain magic gives additional counterplay to this, which is very powerful. It is of course, dependent on your opponent, if i am fighting dwarves it is the first one i am trying to kick out, but i think it is very strong against opposing wizards, And, as i said before, walk between worlds is unique in the sense that 8th is a movement based game, it lets you do things that the rules generally not allowed and that opponents often miss. Yes, i just walked trough that building you anchored your battle line around, what now. Another great spell like that is arcane unforging, many opposing builds just stop to function if they lose one magic item, making this also a spell with compounding advantages. You just get so much of HE cheatyness in this lore, it is all pretty exploitable. 

Went on a bit of a ramble there, but it comes down to: I agree that signatures are better on turn one, but due to spell swaps, i think you can create compounding advantages that really hurt your opponent in the later turns.