I think you’ve already identified what is pretty much the only relevant upgrade option.
Even if you had the hardware and heroic rework skillz; BGA-559 Atoms never got any faster than the N2800, which isn’t terribly fast(and that’s only if the CR-48’s N455 was set up to use DDR3 rather than DDR2; the fastest DDR2 BGA-559 is the whole-200MHz-faster N475).
You can get mini-PCIe to PCIe 1x adapter cards; which are fun toys; and(assuming no ugly gotchas exist somewhere in the firmware or something) could theoretically be used to add a hilariously overpowered GPU; SAS HBA, or similarly ludicrous pairing for an anemic Atom; but it won’t be much of a laptop at that point.
If you confine yourself to things that actually fit in min-PCIe slots; there really aren’t many features you can add. You already have wifi and a cellular modem, which covers the vast majority of the world’s mini-PCIe cards; you can get RS-232, GbE, SATA, and TV tuner(albeit often obsolete NTSC gear); but those all require additional cabling hacked into the chassis and aren’t obviously helpful to the cause of an anemic laptop. The h.264 decoder cards, while they only make a difference in that one use case with suitable application support, are more or less the only mini-PCIe devices that can take a ‘system not powerful enough’ and turn it into buttery smooth playback.
As for software; I suspect that Android won’t save you. Aside from being pretty awful as a mouse and keyboard OS; it isn’t really any lighter weight than ChromeOS or your minimalist WM of choice and a standard Linux distribution. Possibly even heavier. It’s not as though Chrome on Linux gets faster when you add a bunch of freaky not-quite-Java stuff to the party; and unlike assorted awful phone SoCs, the GMA3150 has pretty robust(if spartan, and just plain not very fast, since it’s a low-clocked GMA950 descendant of no particular impressiveness) support for X; rather than some Android-only binary video driver with hideous(looking at you, GMA500 and GMA3600, you PowerVR-begotten hellspawn…) or nonexistent support for normal Linux video options. I’d only touch Android if you have a specific desire for some Android application