If the aircraft is lighter than gross weight, the actual Vso is much lower than the book figure. In that case, on very short final, I will be bleeding off airspeed as I start to round out, and if I’m alone or with only one other person in a typical 4 or 6 passenger GA single, our airspeed should be significantly lower than the published 1.3 Vso.
Some thoughts.
(a) stall speed ratio will vary near enough with the square root of gross weight ratio
(b) before playing with numbers, make sure you are working with CAS rather than IAS to avoid any problems with near stall PEC variation which can present a problem on occasion.
(c) I wouldn't necessarily use terms such as "significantly" lower - just do the sums.
(d) be wary of power (thrust) effects when comparing what you might be playing with in the aircraft with what the AFM/POH might have to say. The certification flight test requirements are fairly specific and thrust can play a significant role with the numbers
(e) Vso doesn't vary as it is a specific certification thing.
If an angle of attack instrument is installed and used, which in effect compensates for weights lower than gross, the approach airspeed, both on long final and on short final, will be lower than book figures, sometimes dramatically so, especially in airplanes with a large W&B envelope. That’s because Vso is much lower.
And ..
(a) keep in mind that you may be playing with varying the design rule book for your jurisdiction should you intentionally come aboard at speeds lower than what the AFM/POH suggests. Potentially, that could present a legal concern in the event of a mishap.
(b) I presume that the envelope reference is to CG range ? I can't immediately think of any reason why the GW range might have a material effect. For the CG range, the AFM/POH stall figures (and the usual operational figures based on the stall data) will be for forward CG as the stall speed varies somewhat with CG, due to tailplane forces coming in to play. Vso, however, is a certification animal and doesn't vary with CG, while the real world figures you might see on any given day may well show a measurable variation. Again, do be circumspect about playing outside the AFM/POH ambit due to the potential for legal censure in the event of a mishap.