Primarchs and Lords of War under 3000 points

[Valkyrion]

Context: 
Me and mine play one game all night. It's just how we do it, with plenty of beer, pizza break, an hour on a spotify playlist and so on, so a 2000 point game will take us 6 hours. This is fine, it's how we like it.
So instead of following the usual rules for Primarchs and Lords of War, we're thinking of the 2000+1, or 2000+2 rule, where we play a 2000 point army plus a primarch each, or plus a lord of war each, or plus both each.  To our way of thinking, this is like playing a 2000 point game but with one extra model or two extra models. In order to field a (500 point) Primarch and a (700 point) lord of war legally, you'd need over 4000 points of other units, turning it into a game beyond our means.

Would such a game just boil down to which primarch and/or lord of war did better, thereby rendering the normal 2000 points moot and the whole evening a waste of time?
Are there any house rules you'd perhaps suggest?

For examples sake, lets pretend it's Imperial Fists, Rogal Dorn and a Falchion vs Death Guard, Mortarion and a Warhound, because that's what we've got. 

[Noserenda]

In theory as long as the points match up it should be fine?

[SkimaskMohawk]

Whenever you're taking a relative small point sized list and dramatically inflating its points cost through two or three more units, then it tends to hinge on those units performance.

If I spend 500 points on a couple of rapiers with fire and shatter, a couple of javelins, and a command squad while my buddy takes angron, he's either going to plow through my stuff or get stuck with too many points in one place. If we both take a primarch, then it will probably hinge on which one generates more value. But hey, maybe you guys want to speed things up a little bit lol.

[Brother Kraskor]

See how it works I guess and then consider house rules if it's too unbalanced? Like maybe the Primarch/LoW can't arrive until turn 3.