Having done a crap ton of No Chill batches in the last year or so, I'm sort of starting to question how the "transfer" addition contributes to flavor and bitterness.
The conventional wisdom here is that a transfer addition counts as a 15-20 minute addition. I planned my beers accordingly. HOWEVER, for me, I noticed that my beers were not quite as bitter, as I'd hoped, nor were they quite as hoppy either, even with "big" additions at transfer. I suspect there are a ton of factors that come into play here, including length of time for your cube to cool down, if the hops are free floating in the cube, the alpha of the hops, etc.
The end result of my personal brewing (YMMV) is that transfer additions contribute the flavor of a 15-20 addition, but oddly enough, do not contribute ANY bitterness at all. Observe my recent experiment below:
5 gallons - ESB
10lb Pale Malt
.5oz Magnum @ 60 (bittering)
3oz US Goldings @ transfer (flavor) (no hop bag/sock, left in cube for 2 days before pitching yeast)
WLP 007 Dry English Yeast
Mash at 152F
OG: 1.050 - FG: 1.015
Result: Beer tastes slightly SWEET, not very hoppy, but has a nice, mild hop aroma & flavor reminicent of "flowers" (not surprising since its Goldings I'm tasting)
IBU if transfer hops count as 1-5 minutes: 28
IBU if transfer hops count as 20 mintes: 40
No way this is a 40 IBU beer, it comes across slightly sweet & mild.
SO, your millage may vary, but for me, I count transfer additions as fairly ineffective 15 minute addition when it comes to flavor, and as a 0 or 5 minute addition in regards to bitterness.