I think that upgrading warp prism speed can be a good thing ONLY if warp prism shield is decreased
Not going to comment on Warp Prism speed directly.
But there's no way under any circumstances that the Warp Prism's shield should, or will, be reduced again.
The reason it was increased was because the fact that the low shield made the Warp Prism useless - very easy for its hull to be damaged, making it incredibly risky to transport units.
Terrans can repair their dropships, Zergs make tons of their dropships. Protoss dropships were out in the cold without the ability to be safely reusable - which is what the shield buff did for them.