You don't say where you sail (or plan to sail) and what kind of boat. In general I would question the need for a storm jib at this stage of your sailing life. We sailed 20,000 miles in the last two years and never thought of using either our storm jib or storm trysail.
If your 150% genoa is the only headsail you have other than the tiny one, ask people who sail in your area with similar boats, how large their gennies are. Most people tend to use something around 135% but a 150% would be fine if you are in an area that does not get a lot of wind.
Something else to think about is the difficulty of changing jibs when it is blowing. The sail that is coming down tends to fall partly in the water (not a good thing) and it can be tricky forward if it is rough, which it tends to be. If you really want a storm jib then your sailmaker's idea seems a good one although I have not seen such a sail. We accomplish the same task by having an inner stay that we can hoist our hank-on storm jib (or staysail) to.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.