" Wouldn't it be better to have another envelope or a polyphonic LFO rather than having to sacrifice VCO 2 to achieve the same thing? "
No ! The limitations of this synths ( P6, P5/10 also) are part of their magic.
Use everything : Pressure, Wheel, Pedals (!) and knobs and the X-mod gives you all, no other synth can do.
First i looked to the LFE-Expander to... but for me, there´s no need for.
The simplificity (right word, i´m from German?), directness and limitation same necessary for this synths like their sound himself...