There are a lot of factors in all this for an underquilt.
Fit is very important and the loft you measure when it is laid flat on a table is not necessarily the loft you will get when it is attached as an underquilt. There are at least 4 components to fit-- the size of the underquilt, the shape of the underquilt, the suspension system of the underquilt, and where you lay compared to the size, shaping and suspension system of the underquilt. In general, wider underquilts that provide a wider range of sleeping positions need a more sophisticated suspension system to maintain a good fit. This is always an issue but is more of an issue when you want to lay diagonal to the right or left or down the center of a hammock. If you always lay in a fixed position then you can theoretically adjust a simpler suspension system that is tailored to that fixed position.
An underquilt will have a base comfort range which is the upper and lower temperature limits that you are comfortable using it without additional efforts, such as supplemental insulation and venting. How uniform the insulation is distributed when you are using it has a lot to do with the comfort range of an underquilt. If you have a cold spot under your tush, then the insulation is not distributed as well as you would like and the comfort range will be reduced. Cold spots also mean that you will have warm areas when the temperatures warm up.
And as nice as extra loft sounds when cold weather is here, it will not seem so nice at more moderate temperatures. There is also an upper temperature limit that you will be able to comfortably use any particular piece of insulation. If you get carried away and use more insulation than you need at the lower temperature range then you will also will also penalize yourself in warmer temperatures by unnecessarily lowering the upper temperature that you will be able to comfortably use it.