There is a similar interchange in my part of the state...I40 at US 321 that does seem to be under some stress. I would be curious about the ADT of 321s to 40e. I know the merge of the CD lanes onto 40e do become congested at turning rush hour.
Here's the AADT map with the volumes: https://ncdot.maps.arcgis.com/home/webmap/viewer.html?webmap=ff72d8f962bf40ac8973669fcdc63380
The volumes in Hickory on the loop ramps vary from 2300 to 7400 (SB to EB), so still under what I'd consider a problem level. That stretch of I-40 is getting close to needing six lanes; I could see auxiliary lanes being added between some interchanges as well that aren't currently there as part of that project, including between 321 and Lenior-Rhyne Blvd (the EB on-ramp from the 321 C-D is at 14K, which is a pretty heavy merge.)
I know that I-40 project's been studied, but all I see in the area is a pavement rehab one for now.
A 1/4 mile long exit lane leading up to the westbound exit for US 321 would be nice during the next 10+ years it takes for the widening project to move forward.
What is the average volume threshold for a multi-lane exit with option lane? There are a lot of newer projects with these (Goldsboro Bypass, CF Harvey Pkwy, Asheboro Bypass, Greenville SW Bypass, Monroe Bypass), but then older interchanges with a lot more traffic still have a single lane freeway-to-freeway exit (e.g., US 74 at I-26 EB and I-85 SB). It would be a relatively easy and effective upgrade, but it never seems to be considered unless it's on new location or a widening.
I'd probably want the deceleration lane approaching 321 to be longer than ~1300'. Would be nice as an interim, not sure it can happen. (It's only money.

And a thousand other places that need improvements.)
For ramps, a single lane can handle 10K+ ADT volume. What you're seeing on newer freeways with option lane exits is often driven by weaving issues. If there's an upstream onramp that creates a lane and a downstream off-ramp closely spaced, our analysis may show that the weaving distance between the two is not enough for a simple weave. We'll add the option lane more for the mainline exit rather than for capacity. Of course, the off-ramp will need two lanes for this. Changing to an option lane isn't that difficult, the expense is in widening the off-ramp. (There are some grading details with paving a gore that one needs to pay attention to.)
A challenge is signing the option lane. It now requires the huge signs that have the individual lane indications (you guys here know more about what to call them than I do. That's not my area of expertise, or interest beyond what I need to work with in designing interchanges.) I preferred the older signs that allowed us to indicate that a lane could exit but wasn't an "exit only" lane. The MUTCD stopped that and causes us to spend a lot more money on overhead signs than we used to. (Personal editorial, not a department position.)
For a cloverleaf, the weave area tends to break down when the combined volume on the two loops gets over about 1000/hour. That does depend on the distance between the two gores. I don't think you'll see many cloverleafs in the future on NC highways. At service interchanges (surface roads) the most efficient higher volume interchange is a parclo B (off-ramp loops from the freeway) where the interchange ramp intersections are leftovers/right-turns (they are RCIs/superstreets/RCUT intersections) and we're converting some exiting ones to that. One recent example would be at I-40 WB and US 70B in Garner (exit 306) where the on-ramp loop from EB 70 was converted to a leftover. That was a safety issue on I-40 there. To your point of "easy" fix, that was done as part of the widening project, so there was a funding source to do it.