Performance fit is definitely less than a finger and a half.
(I'm not a bootfitter, and can't know your own particular situation and experience, so if I'm too obvious or off for the OP, apologies.)
Sort of obvious, but a performance fit would indeed
usually be more like 1/2 to 1 finger (unless, for example, one's foot sizes are very different?). In addition, probably there would be some adjusting, grinding and/or punching for most people: in most cases, you are sizing down a size or so, in stiffer, less yielding material.
In my own case, a pair of very experienced bootfitters steered my very much in a wrong/deadend direction (into some very pricey mistakes), for me and my feet in particular, by saying to go with what initially fits best. There are other considerations, and lots of routine adjustments that can be made by top bootfitters.
Usually, for performance fit, one needs to size down a size or more and then make adjustments, or otherwise the fit on the snow may be too loose. (The right bootfitter can guide you here, if you don't have the experience yourself.)
In addition, even if it does fit perfectly at first, if the boot itself is too soft (in either forward or torsional flexing) for your type of skiing, or part of it, then it will be money wasted, better skiing lost.
Similarly - again as you may know and have made allowances for, at least the two Lange boots you mentioned have some performance limits, and are usually/often not really performance fit boots: I find them softer flexing and softer torsionally than is optimal for about half my skiing (slalom-like turning, deeper powder, rougher soft snow conditions, ice and groomer charging, race type carving). As a result, the RXes, in particular, are often stiffened up with additions and mods, such as replaced liners and changed out (Booster) straps, with straps placed under the cuff (which makes the flex stiffer and more snug-fitting/flexing). There's a whole bunch of bootfitters who routinely do these mods to the RX 120 and 130, and ski on those mods themselves. (Please, someone, correct me if I'm out of touch with changes in just the past year or so.)
On the other hand, for "in between" type skiing, more relaxed; casual longer or relaxed gs and in between turns; casual all mountain and resort powder cruising at medium to modest speeds; and/or a pivoting and slarving style in general, including a pivoting style in bumps, then the RX 130 and XT 130 can work wonders.