If anyone would be kind enough to assamble them in such a way I can bload them from basic and use them with DEFUSR, we could have acces to the GFX9000 graphic commands from Basic
how many kilobytes would that need? 
A hand full of gosubs that do some OUT for vpoke, copy, set_scroll can serve a whole game.
The scroll X register bits are laid out unhandy?
This is NOTHING compared to all the burden on MSX1 and MSX2.
The many problems discussed in those threads dont happen on 9990.
Too many messages to answer, I will try to be generic and simplify:
-I store as much data as possible in Vram. The maps of all my games are on Vram. It is the slowest way (to read from vdp both the map and character's/projectile collisions) but since I have so little ram, I have to use vram even if it is slower.
-GFX9000 has 512k of vram (4 times more than a regular msx).
How many kilobytes would an ASM routine need to handle the necessary v9990 commands?
Less than having to store the original basic code and the compiled basic code for such routines, since we would have it compiled already on ASM, so we can avoid the uncompiled basic version of the code in the ram.
Also, using a=usr() takes less space than a whole bunch of OUTS so, we are saving memory there as well.
An ASM routine to handle gfx9k would indeed save memory, as you can see.
Really? That's a HUGE downside!
Clearly the worst feature of Turbo Basic, duplicate program in memory 
@Kai I'm having a hard time trying to convince myself to work with Turbo Basic after knowing about the memory constraints you and @Grauwl pointed. Its a complete nonsense having the same code twice on a 24k work ram. That's why I insisted on Konamiman, because his NestorBASIC bypassed this limitation by allowing the user to use Memory Mapper on BASIC programs, and it could be upgraded with a V9990 plugin like the MoonBlaster replayer and OPL4 plugins.
You are wrong, Nestor Basic don't allow to bypass this limitation. The code itself remains with 24Kb available and is duplicated in memory, so the limitations are the same. Nestor Basic allows you acces to all segments of mapped memory in a linear way to store data or to store other programs and jump from one to other (it's not much different to load different engines since only one can be ran at same time). This was the main cause why I left Turbo Basic first and Nestor Basic after.
However take something in consideration: are you alone to make a game? How much complex your game is going to be? If you are alone and are not going to make a complex game, perhaps BASIC is not a bad choice. All depends the game you want to make.
If you program in C or Pascal, you run under DOS and you have more than 50kB of memory...
I think that C/Pascal will be the best alternatives for program MSX in medium term 