Why would it be negligible?
We don't know what design and size Thunder CFT's would be but using the same ratio as for F-16, Thunder CFT's would be about 60% of the underwing tanks, and then underwing pylons would carry additional armaments so the impact on the range would be quite significant i,e. less fuel, more drag due to CFT's (howsoever negligible, it won't be zero) and then the additional weight of underwing armament.
F-16:
Internal 7,160lbs
CFTs 3,000lbs (36% of underwing)
Centerline 2,045lbs
Underwing 8,180lbs
Thunder:
Internal 5,130lbs
CFT's 463lbs (36% of underwing)
Centerline 467lbs
Underwing 1,286lbs
What?
Use of F16s underwing and centerline fuel carrying capacity here is not relevant.
Wouldn't the only thing relevant be the
capacity of fuel tanks currently being carried by JF17 vs the
fuel carrying capacity of potential CFTs on JF17?
It is the difference between the two that we are interested in. Currently(according to wikipedia)...the JF17 optionally carries an 800 Kg(1764 lbs) fuel tank on the centerline hardpoint...
...while the two underwing fuel tanks can be either the 800 Kg(1764 lbs) or 1100 Kg(2425 lbs).
So if instead of the two underwing hardpoints carrying fuel tanks...the two CFTs are to carry it...then we will need to know the capacity of the supposed conformal fuel tanks of JF17...which don't yet exist...hence why I had used F16 as an example here. The reason for using F16...as opposed to some other aircraft is that F16 is similar in dimensions to JF17(with JF17 being slightly smaller). F16's CFTs both combined carry 3000 lbs of fuel...so that's 1500 lbs each.
Now circling back to the JF17's underwing tank of 800 Kg(1764 lbs)...that 1500 lbs isn't too far off.
As for the drag induced by the extra munitions carried instead of the fuel tanks...well in absence of those munitions...when the fuel tanks are carried underwing...they also add to that drag...
...to remove those fuel tanks from underwing and attach munitions like an ARM missile...it wouldn't really add too much of an excessive drag penalty.
The biggest two drawbacks are...
1) not being able to use the 1100 Kg fuel tanks...the centerline can only carry 800 Kg...probably bcuz of ground clearance issue. So if the underwing hardpoints are not utilized for fuel tanks...that 1100 Kg fuel tank can't be carried.
2) the second main drawback is the added overall weight. Still hanging munitions...while also carrying extra fuel in CFTs...as opposed to only hanging fuel tanks and not using CFTs.
A minor drawback is still the slight difference in fuel...1764 lbs(underwing if using 800 Kg tank) vs the 1500 lbs estimate of the CFT...
...a difference of 264 lbs times 2 = 528 lbs.
Another minor drawback could be the drag induced by the CFTs...but that remains to be seen. It would depend on the aerodynamic characteristics of the CFT...it doesn't always have to impose a drag penalty...in case of the CFTs on F18 it creates lift IIRC.
In any case...CFTs on JF17B make sense in the wild weasel role bcuz of the two hardpoints that are freed up...other than that role it seems to be doing fine without CFTs.