The main problem of spells in h3 is that good spells are on level 1 and 4 and 5. The second problem is that level 4-5 are kind of pointless even in longer MP games due to the fact that you have to choose magic school on you main hero before you know what 4 level or 5 level spells you have in your guild. Thus it is a big gamble to invest resources in high level mage guilds.
This change doesn’t really fix any of that problems.
Moving Mass haste and slow to higher levels on the other hand would change the gameplay by actually destroying balance for MP . Right now because of low level you can almost always get either slow or haste. If it is higher level one game you will get it one not. Considering how big difference having or not having mass haste/slow does it is a win or not win just because you didn’t get this one spell .
Considering how MP tactics centers around both mass haste/slow I do not think any change to them will be widely accepted by players…