It would be nice to have a C5, but it isn't necessary.
Firstly, if I'm really exploring, then I'm only jumping small distances. With short jumps, a C4 fuel scoop has finished before the honk. It's getting out there in the first place that needs the jump range and drinks the fuel. And I do a lot of that because I tend to explore for a few days and then go back to the bubble.
That sort of jump is generally pretty much in a straight line, which means the next jump vector is hidden behind the star when you jump in. Getting around the star with no more than 65% heat generally fills the fuel tank on anything other than an M class star. With those I can find myself perpendicular to the exit vector while still scooping. In that case I throttle back for about two seconds; that is literally all it takes to fill the tank. I can then turn away from the star in the most efficient direction to reduce heat, ie directly away from it, and hit the hyperspace button as soon as the star turns yellow.
With only a C4 scoop, the DBX requires that you treat scooping as an activity. You can't just fly close to the sun on the way to the next vector. To my mind that is more an adorable foible than it is a fault.
I looked into the practicalities of making one of the C4 slots into a C5 a while ago comparing it to the AspX and delving into the displacement mass of both ships. From a volume point of view, I've said before that the DBX is too small already for the stuff it contains. But, from a mass perspective, it has plenty of budget to play with.
I said:
I've just run the numbers. Obviously the Asp Explorer has a more dispersed frame -- it's medium sized -- but it has almost the same hull mass as the Diamondback. It's 280 tonnes rather than 260 tonnes. But the total equipment mass budget for the Asp is 362 tonnes to the Diamondback's 220 tonnes.
That was simplistically adding the cargo-mass equivalent for each slot. That fails for some non-cargo slots -- a power plant for example is often heavier than the slot it occupies. Even so the AspX exceeds its hull displacement mass and the DBX does not. If it was impossible for the DBX to have a C5 slot then the AspX is already overloaded.