I don't know that's it's a slam dunk to refi. You'd have to weigh the costs of the new loan w.r.t whatever rates are in 1.5yrs. It will likely save you money, but it doesn't mean that it's an obvious choice. I would continue to save up as much cash as you can and get an appraisal when you're ready and then see how the numbers work out. Assuming that you probably have decent incomes, you are saving money pre-tax now in a potentially higher tax bracket than when you retire and the mortgage is paid off... iow, you may be better to save taxes now and pay a bit more mortgage interest, than to pay taxes at higher rates (by stopping 401k contributions) and reduce your mortgage interest (by way of saving up 20% to refi to a lower rate). If you save $10k per year on taxes by saving into your 401k, yet pay an extra $4k in PMI and $4k in mortgage rates (vs refi) and would otherwise pay, say $2k on the eventual withdrawals of the money in 30 years at retirement, then those numbers are a wash (by design). I don't know if that's the case, but that's how you could look at it and plug in your own assumptions. I probably would only refi if you do so without reducing 401k savings. There's no obvious answer, which is good, because you probably won't choose a path that had a huge consequence.
PS - I'm 4 yrs into an FHA loan at 4.5% with about a 90/10 LTV and have cash to refi if I wanted, but for liquidity reasons - job change, I chose not to because the numbers didn't scream advantage - it was a small advantage, but not large.