RAM expansions are fine if you: A) already have one, and B) don't mind the decrease in code execution speed that you will see on the GBA bus.
The speed decreases come from several factors related to the GBA bus. For starters, the GBA bus is very much slower than the RAM bus. The RAM bus has essentially 1 or 2 wait cycles for all accesses (I do not know the exact number), which means the CPU accessing RAM has to pause for 1-2 cycles while it fetches the data. The GBA bus, however, has a configurable wait state setting of 6-18 cycles for the first halfword access, and 4-6 cycles for the second halfword (used as the upper halfword for word accesses). Meaning you're going to see a 10 cycle delay for every ARM instruction (or 6 cycle delay for every Thumb instruction) on the GBA bus at a minimum.
Things are further complicated by the fact that the ROM/RAM chips used on the GBA bus may not be fast enough for the lowest wait state settings! On top of that still, while physical memory may have a 1-2 cycle wait state for all accesses, the ARM9 CPU has an instruction cache which can be used to execute instructions rather than reading each from memory every time. The use of the instruction cache brings it down to 0-1 cycle wait.
That said, there's also a data cache on ARM9, which works quite similarly to the instruction cache, and helps reduce CPU overhead. The GBA bus cannot be cached.
And finally, even with Thumb instructions on the GBA bus, you may need 2-3 Thumb instructions (or worse, PC-relative loads!) to match an equivalent ARM instruction, slowing it even more.
So yes, it will work! But it's far from efficient. I would not have gone to the trouble of designing an additional LPT port for Xport (it already has one... but it's only 4-bit and the protocol sucks) if efficiency were not important. And I believe USB will be even faster. (The 8-bit LPT port is currently able to do about 2.1mbit/s (~15 seconds for full 4MB RAM dump) with ARM instructions in cached memory on ARM9 with a fully software-based protocol. USB has a hardware-based protocol.) So you will have the option to use external media on the GBA bus if it's all you have or if it's absolutely necessary for any number of reasons, but someone has to work on them. It probably won't be me.
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