No your aren't missing something.
You are beginning to get your arms around some solid facts, except that pressures are lower and velocities higher with lead than with jacketed. Normally, LBT bullets will not seat as deeply in the case as jacketed of the same weight, when set up to function smoothly in an auto, however the WFN may go in a bit deeper, which will raise pressure and velocity compared to the same lead bullet seated to the exact same depth as an equal weight Jacketed, seating depth being referred to in this instance being the depth of the bottom of the bullet.
My statements above are dependant on using LBT bullet lubricant only. The ACP works at low pressure making it quite forgiving as to lubrication quality. Many lubes are of such poor quality, (provide poor lubrication) that pressures will be as high or higher than jacketed with some loads, with velocities ranging from about the same to slightly faste than jacketed. But ALWAYS, at ACP pressures, using LBT bullet lubricants, lead velocity will be higher/velocity higher than with jacketed, and, if the powder charge is increased slightly so that pressure matches the safe pressure with jacketed, velocities will be MUCH higher. I've chronographed up to 150 fps higher with plain base bullet. Same applies to magnum revolver and high pressure handgun loads, but optimum velocity minimum pressure will be obtained with gas checks when pressures exceed perhaps 35,000 psi.
If you see no evidence of battering of brass or hard slamming of the slide against it's stop, the spring you are using should be fine. Chamber pressure X dwell time at pressure, or pressure curve, are the factors which drive the slide backwards, not necessarily recoil as calculated by bullet mass and velocity. In other words, if pressures are lower than with jacketed, slide action will be lower, even though bullet speed is higher.
