I agree with @Dan H., Prop 13 should not be considered in your calculations. The key is that you should buy an asset that has negative cashflow. Don't count on appreciation. Using the 200k from the refi to invest in properties that have better ROI will only subsidize the loss you will make on your old home. Which would you rather have?
1) Selling your house and investing the 280k in out of state cashflowing properties. If you invest in Memphis or Cleveland for example you could buy and finance 14 houses (portfolio loan) would get you $3,500 net cash flow. Return on Equity: 15% (3,500/280k)
2) Refi your house and investing 200k in the same types of properties above will get you 10 houses at $2,500 net cash flow. Take away the negative cashflow on your SD rental of $1,300 and your net cash flow on your portfolio is $1,200. This is ⅓ of the net cash flow from option 1. Return on Equity: 1,200/(200,000 + 125,000) = 4%