obviously, apart from destroying the 'muskets in rain' "feature" in Mongols, raising musk costs to 400(!)koku would be welcome...

Heavy Cavalry...
-1 defence, +2 attack, +1 Armour (on original stats) would be my choice...
I like the YariSam cost up and stats up...however I would go further (in a perfect world)

Tone guns down to 1/2 present strength and retain a 250 koku price-tag, raise all Samurai koku costs by x2 and half all Ashigru koku costs, leave the Ashi as it is but bump all Samurai stats to reflect a x6 increase over their previous stats (leaving a 150% advantage over ashi to make up for cheaper armour/weapons on the ashi)

In a 4000 koku battle (which I would suggest as default for these stats), the player chooses an army which *has* to be mostly Ashigru otherwhise they simply won't be able to afford the units, however Samurai are *better* for cost than Ashigru, somewhat evening out the Teppo Ashigru situation.

Just my thoughts...