Does anyone know what the approximate strength of the pieces in Grand Chess and Seirawan Chess is?
Is it possible to assume that the difference in strength between a queen and a hawk is the same as between a rook and a knight? Can we assume that the queen is slightly stronger than the elephant, and this difference is approximately equal to the bonus for two bishops?
How does the strength of the pieces change when moving to the 10*10 board? Are the long-range pieces getting stronger or are the knight and king getting weaker?

I remember seeing somewhere that the bishop+knight combo is a 7 and the rook+knight combo is an 8.

There is a game called “Almost chess”. The developer (Ralph Betza) writes that in it the queens “have the combined moves of rook and knight”. And a variant is possible where one side (black is implied) has normal queens. Ralph Betza writes that “it is an even game, even for masters”. This, I believe, means that the difference between these “queens” is comparable to the advantage of the White`s right of the first move. I.e. no more then a few tenths of a point.
(This applies to the 8*8 board.)

With the larger board size of Grand Chess, I have to believe that the knight is slightly weaker than normal. Therefore:

Grand Chess:

Marshall: 8-8.5
Cardinal: 6-6.5
Knight: 2-2.5

Seirawan Chess:

Elephant - 9
Hawk - 7

However, I'm far from a master strength player. I'm mostly just thinking logically.