If you aren't looking for more vacation time but want access to star options, I wonder if they would take one of the sdo units in exchange for Westin flex?
If you could get $25k or so credit plus $10k new money you could get a 81k annual westin flex (or maybe a bit more?) Plus retro one sdo 148k. That doesn't add a bunch of MF, gives you star option booking, and doesn't hurt too bad on resale.
The problem with this is that you are giving up one of the most cost effective SO generators in the system. An SDO plat will cost about $0.008/SO in MF per year (it's low enough that it might even make sense to convert to Bonvoy!). Westin Flex runs just over $0.02/option. For comparison purposes 2BR WKV Plat is $0.011/SO. The SDO plats also sold for much more originally and it would be a waste to turn them back into Vistana. The original owner of our SDO EOY 2 BR plat paid $32k from the developer - I imagine an annual would be almost double that.
Current MF's are just over $2400/year for the two SDO.
1 SDO 2BR Plat + 81k WFlex annual is a bit over $2800 MF/year and gives you 229,100 SO/year. Not quite enough for two weeks in Hawaii every year. Blended cost is $0.012/SO. WKV option is slightly lower per SO.
2 SDO 2BR Plat + 81k WFlex EOY would run about $3200 MF/year and give an average of 336,700 SO/year. Enough for at least two weeks every year and a bit. Blended cost is $0.0095/SO which is still better than WKV on a SO basis.
If the WFlex package would be too much to use keep in mind that you have 8 home resorts that you can rent out of. You could book a 1BR week in Maui using WFlex and I'm sure you could probably get $2k easily which would cover the MF (the one downside is there is no OF views in WFlex and event weeks in Hawaii are not possible).
If you are going to own this for any a long period of time MF costs will matter (I'm looking at a minimum of 25-40 years of vacation left - assuming I reach a normal retirement age so keep this in mind when reading my advice). The other thing to factor in is that with both SDO units being in the network you may not need a separate II account for trading.
Of course MVC could come along and blow up the whole VSN system...