(Can't believe this subject has not been covered before now but I did a search all the way back to the year 2000 and found nothing.)

The B737-800 at Max Landing Weight (66.3 tons):
Vref 40 = 142 kt
Vref 15 = 157 kt

With regards to Aircraft Performance Category for circling approach purposes, the B737-800 comes under Category C during normal (Flap 40) operations. However when the operations are not normal such that Flap 15 must be used (Alternate flap operation, one engine inoperative, etc) shouldn't the crew then consider the aircraft to be Category D with the subsequent increase in circling minima and circling area?

The Australian AIP says:
ENR 1.5 HOLDING, APPROACH AND DEPARTURE PROCEDURES
1.2.2 An aircraft must fit into and be operated in accordance with the requirements of only one category. An aircraft:
a. may not reduce category because of reduced operating weight, but
b. must incease category when actual handling speeds are in excess of thse for category (based on Vat) detailed a sub-section 1.15
(Vat = speed at the threshold)
Cat C Vat = 121 - 140 kt
Cat D Vat = 141 - 165 kt

I know that it also says that Vat is equal to 1.3 times the stall speed, and that the Flight Crew Training Manual only shows speeds to stick shaker and that stick shaker has a margin from memory of 7% above the actual stall speed (which I can't find written anywhere anymore). But even with this in mind, the B737-800 (and B737-400 for that matter) still becomes a Category D aircraft when using Flap 15 for landing.
It's just that in all the years I have flown twin jets, this subject has never been mentioned.

What are your thoughts please?

1.2.2 has answered your question. It is a cat C airplane normally at normal landing flap Vref and max certified landing weight, but note that it has to be capable of circling at the specified speed as well as landing at the stated Vref. If the Vref happened to be at a Cat B value at some very light weight on a particular occasion, it still could not be legally operated to Cat B. However, if it was in an abnormal configuration carrying less flap so that either the circling or Vref speed went above the Cat C limit, yes, you most definitely do need to apply Cat D minima or risk the ire of the subsequent court of inquiry if it goes wrong and they start looking for a scapegoat. It's not about practicality as a few more knots probably would not put you into the trees unless you were really wide on the circling - it's about a fine point of law.

Blip,
Your answer can be found in the Jeppesen manual under Air Traffic Control on page 212 (Although I an looking at an out of date copy at the moment).
Under PANS OPS 4 each cat of aircraft has not only a range of Vat but also a range of speeds for initial approach, final approach, circling, initial missed approach and final; missed approach. Please note that the data below is from my out of date page so should be used as an example only, but it is worth looking up next time you climb in the cockpit.
Cat C are;
Vat 121/140
Initial approach 160/240
Final approach 115/160
Max speed for vis man 180
Max speed for initial missed app 160
max speed for final missed app 240
Always check the bottom left margin of any Jeppesen plates you use to see whether the procedure is based on PANSOPS 4 or TERPS because the procedures have very different obstacle clearance margins. Particualrly when circling as, from memory only so please check before using these figures, PANSOPS allows Cat C to circle out to 4.3nm whilst TERPS only allows 1.7nm.
Although reading the Jepps can be tedious, there is some good gen in there.
If in doubt, go around and stay safe.
MM