That infant insert adds a LOT of height. I tried the MR out on the bench, and figured my little guy had just under an inch. Went home, debated on it, read that it tended to increase installed and then decided to buy it (with the intention to pass down to baby in just a month or two... and also knowing I could use FF if it came to that.)
I took it out to the car today, took out the padding, fixed the straps, etc, and although it wasn't 100% installed, I got 3" on the tape measure. Yeah
Little guy is 2y4m, 37 or 38" standing, and very solid, about 38lbs. He'll outgrow it by weight before height, I bet... although summer is coming, maybe I can slim him down a bit! Ha! I don't have a torso measure, or a seated height, but I can try to get those tomorrow if it helps. (The amt of room njkj has looks about right to me.)
My rationale for getting it for little guy even just for a few months of use was due to the amount of driving we do in our summers, and wanting him RFing... and even with that, I wouldn't have done it if it wasn't for being able to pass it to the babe. So if you're *only* buying it to squeak out a couple more months of RF, it may not be worth it. (Though I just noticed you said lil bro could use it, so your situation sounds similar to mine.)
HTH