I found in the main manual (version 14) pages172-173, how an expression pedal set to control volume could provide you with a post amp pure boost without eating up one of the effect modules, IF you wanted to utilize a volume expression pedal for this purpose, as it lets you set minimum and maximum values per rig, which includes boosting higher than unity, if I understand correctly, but then that would have to be a separate thing to control from toggling a drive effect.
IF you don't use morph for other things, you can achieve what you are going for in a superior way than having a post fixed effects pure boost. A forgotten feature is that you can simulate toggling effects on and off by morphing their mix from 0% to 100% (Or 100% to 0%) If you were to, say, morph a Kemper drive in module B from 0 to 100, it will sound like it's off in base state. (You won't be able to hear a difference toggling it on and off if its mix is set to zero). So using morph you can simulate toggling the drive effect on by morphing its mix while simultaneously morphing the volume control up in the amp section to provide you with the volume boost you are after. Even better you could also morph any other controls in the amp section that help your lead sound pop, while increasing the amount of noise gate or lowering the compressor, or whatever else.
A more ergonomic option (if you don't need the in-between morphs an expression pedal provides) is to trigger morph with the five Rig selection foot switches (see manual page 316) Set the rise and fall to zero in the Rig settings of each Rig. This let you instantly jump to from the base and morph by repeatedly pressing the same foot switch you used to select the Rig, (you'll see the LEDs on your Stage or remote indicate whether you are in base or morph state!) So if you are in Rig 2, pressing the Rig 2 foot switch again and instantly jump to your lead sound. If you want your to get to your lead sound in the morphed state of Rig 4, just double press the Rig 4 foot switch. The best part is that you don't have to switch rigs, then lift your foot to press a back row effect switch, like you would if you programmed one of them to toggle on a kemper drive and a pure boost. In this way, in a single performance you can instantly jump between five fully independent Rigs and instantly access a boosted version of them by a double press on the same footswitch. Then you still have all four effect foot switches to toggle other effects in each Rig.
However, I still 100% agree that a simple pure boost is a no-brainer in the post fixed effects, as being able to add a little volume boost to a lead sound would be a welcome feature. (While lowering clean comp helps, being able to fiddle with the amp's gain without worrying about volume balance with other rigs is one of the things I love most about kemper). If it's a matter of preserving as much DSP as possible, Kemper could add a pure boost setting to the post-amp fixed chorus. (so that the pure boost isn't one more independent effect you can toggle in the chain) Then you could leave the fixed chorus depth at 0% and increase it's hypothetical pure boost setting, enabling you to use this effect as a pure boost instead, but without introducing an additional fixed effect in the chain. The reason this could be a logical solution, is because the whole point of offering a post-amp fixed pure boost is because guitarists have other uses for the X and Mod modules than wasting them on a simple volume boost, since these modules are prime real estate for all the advanced stereo modulation effects—including the more advanced choruses that would make using the fixed chorus redundant. However, another way to include a toggle-able post-amp volume boost is to simply add a pure boost parameter to the amp block itself and allow it to be a toggled the same way the freeze function in the delays is assignable. From a DSP standpoint, this volume bump could be processed in mono, because it would be BEFORE the stereo X module, unlike the post amp fixed effects which are necessarily stereo.