Guidelines on sumo

Are there any guidelines that keep in mind how your body is proportioned and what kind of stance should be optimal?

Yes Sir, they are.

But I really do not know how accurate and effective they are.

I honestly do not know anyone using it. The most important guideline is: what style is more confortable for you?
By what type of deadlift do you lift more?

Anyway, let's have a look to the aforementioned crierion (Author Tom De Long, as far as I know).

1. take the height between shoulder and floor (torso +  legs). This is L
2. take the distance between shoulder upper part and fist. This is A (arm length)
3. Rise your leg and misure the distance between upper part of hip joint and upper part of shoulder. This is T (torso length).
4. At this point you have also that L - T = legs length. Let's call it "Le".

With that said, now we estimate the torso/ arms proportion and the torso / legs proportion.

Let's make an example. Tom has T = 52 cm, A = 75 cm, Le = 97 cm.

T/A = torso / arm proportion = 0.69
T/Le = torso / legs = 0.54

Rule states that IF T/A < 0.82 AND T/Le < 0.55 you should pull conventional. It is rather long arms AND long legs compared with torso.

Conversely, if you have long torso, then T/A > 0.82 AND T/Le > 0.55 you should pull sumo (rather short arms and legs compared with your torso).

These are the 2 ideal cases.

For example I fall exactly in the middle (T/A = 0.82 and T/Le = 0.55). Indeed I pulled conventional with good results in the past and now I pull sumo with good results as well.

I think you can make this experiment, but once you have finished, follow your feeling during practical test.

Is there a way to predict if transition to sumo is worth the time?
For measurements i fall in the middle. I tried sumo only 3 times or something like that in the past 12 months, so technique is really not nailed down. One time i tried for a max pull and was sort of 40kg below my conventional max i pulled the same day, while the failed sumo deadlifts did not move a bit  ;D

In regards of comfortability conventional is more natural to me but this is the result of the longer time practicing the lift i would say.

In generally my deadlifting is not as good as i want it to be - still below 200kg and now on my way back post injury and the next competition  is  more than three months away.

The formula provided above makes a lot of sense cause it is based on leverages.  Now we all know that most lifters who do well in certain lifts have advantageous levers for that particular lift, for example most great deadlifters have short torsos, long arms and long legs while most great bench presses have short arms, long torsos and wide shoulders.
However this formula fails to take into account other physiological features such as width of hip structure and thickness of back musculature. 
We all know that sumo deadlifts place more stress on the hip while conventional deadlifts place more stress on the back.  So more often than not, lifters with wide and robust hips can take more work in sumo style while thinner lifters with smaller hips might encounter hip pain/stress deadlifting sumo.  Likewise, lifters with thick strong back musculature might prefer deadlifting conventional while athletes with less back thickness compared to their hip structure might prefer saving their backs from extra stress by performing the lift sumo style and shifting the emphasis to their stronger hips.

Interesting thread.

First let me  tell  you my example, I am a 6ft(183cm)tall, have long arms,  compared to torso,  but even  longer  legs. So although this  formula says me I am suited  for conventional ,  when I am in a "good" starting  position  with my conventional, my torso is  basically vertical, because of the much longer legs. And lower back strength is and was always the  limiting factor  for my conventional, cause I am basically doing stiff leg deadlift when i pull conventional. Even when my shins are not vertical and with knees over the bar.
I  tried sumo,  it was awkward at first, was really weaker,  but built it with  time and  now it  is  better then my conventional. For my build I think it is much safer  to pull sumo for max attempts. Torso angle is higher and load on the back is less.
My humble opinion is that if you have good technique and work on it for a very long time,  almost everybody will have higher  max with sumo,  no matter of the proportions. So everybody should at least try it for a  couple of  months...