This is more of a brain dump than a scientific analysis, but:
40MPH = 34.7Knots, give or take a couple of decimal places. For a 4.5m rib, 60 Hp (even with twins) that could be about right. I have a 5m hull, single 60 which was passing 32 knots & still accelerating when the engine expired at around 5200 rpm (that lamentable story is spread around the site on various posts), so that doesn't seem too far off the mark, give or take a bit of sales pitch.
Now, dropping to a single 30 is probably not too far off my old setup which was a 4M with 25 on the back, and would do 21 knots flat out with a 13" prop. I did upgrade to 30, but never got a run with a GPS before I sold it. Cruising in company it was clocking about 24 knots on the other boat' GPS.
Now, you have twins there, so there is a realistic chance that when running on one, it will be well over propped (especuially if the other engine is dragging) and not get anywhere near max RPM, hence your 13 knots. When you run single, is the other leg in or out the water?
As I said in another post, if you want more forward motion, by dropping in diameter slightly, you can re- load the engine(s) by upping the pitch. Only down side is you loose grip more readily, but with twins you will have far more blade area to play with (area is proportional to radius squared), and so there's a higher chance at least one will keep hold.
You really need to get that tacho wired up before you spend oodles of dosh on more props!