and is one of the sharpest films for reversal
		
		
	 
Note that DR5 ducks the issue by using the qualifier 'for reversal'. Both Deltas 100 & 400 are in the same sharpness range when reversed - which is good 
for a reversal film. I know exactly how sharp both Delta 100 and 400 are when processed conventionally and optically printed or scanned on high end equipment. I also know what they look like when reversed and optically printed (by purely analogue means) or scanned on high end equipment. BTDT. You will think the reversed Deltas are sharp until you see the conventionally processed ones. Or to put it another way, reversal takes the sharpness back to the 50s/ early 60s.
	
		
	
	
		
		
			Double X gives sharp results when reversed and scanned.
		
		
	 
Have you ever processed it conventionally (in a known standard developer) & were the scans done with zero sharpening on high end equipment? As a negative, it should be sharper than 400TX at very low frequencies, but TX is considerably sharper for some time beyond about 20cyc/mm (which will also enhance our perceptions of granularity from more highly enlarged small format negs used in still photography) - and from my experience of 5222, Kodak's data seems to correctly characterise what I perceived the material was doing sharpness-wise.
	
		
	
	
		
		
			No idea if Double X and Delta 100 have DIAR or not.
		
		
	 
DIAR and DIR are couplers. They are a fundamental characteristic of C-41 technology that cannot easily be cross-applied to E-6 and essentially not at all to B&W. A key design goal of Delta technology was to obtain emulsion behaviour in conventional (vs chromogenic) B&W films that could deliver DIR/DIAR-like sharpness & highlight control characteristics.
	
		
	
	
		
		
			it is possible that loss in sharpness noticed by some in Delta 400 slides is due to user error and has nothing to do with DIAR.
		
		
	 
No. It's fundamental to reversal process shortcomings. You can see it with all B&W emulsions if you process them conventionally vs reversal. Look up the MTFs Agfa published for APX 100 and Scala 200x (they were by all accounts essentially the same emulsion, coated at different amounts of silver/m2 - which probably rather confused some who tested them side by side as negs & claimed there was no difference other than the base) and it makes this clear. There are various reasons why transparencies have clung on, but actual 
usable sharpness is not one of them. Rather than obsessing about errant notions of sharpness, accept that transparencies aren't going to be as sharp as neg/ pos & instead let them do what they do well & use them as an appropriate expressive device when needed. A lot of very costly basic R&D was expended on improving the sharpness of reversal materials over the years - and they never managed to meaningfully better neg/pos.