Need help optimizing formula that returns row number

Well, the mmult is probably the biggest killer, as I imagine that is rarely limited to just 25 rows.

Seems like the best way to speed it up is to avoid the mmult, and with that I think you need a helper sheet. The top row would be the SKUs. Under it would be the cumulative sum of parts for that SKU (i.e. recreating the results of your mmult). The good news is that this could be a simple sumif. As a quick example (but I would recommend creating a sheet like this), if cell G40 has "B" in it, then in G41 you type in:
=SUMIF($D$14:D14,$G$40,$E$14:E14)

Drag that down, and you have the results of the mmult for that sku. You can change the B to an A and it will adjust. But, when you create a table like this (on a separate sheet) then you can change your formula to use some MATCH/INDEX lookups to get the value.

Not as dynamic as your method as you will need to add new SKUs to this sheet (or use some other Excel tricks to automatically add them).