I'm a Bulgarian fluent both in Serbian and Slovene. My feeling is that the average objective mutual intelligibility (MI) between these two languages is either similar or slightly higher than the mutual intelligibility between Serbian and Bulgarian (50-60%).
I am using the term "average objective MI" as MI can be very deceiving from different angles and needs to be studied carefully. I would like to bring up two points related to this:
1) Complex syntax constructions are relatively rarely used in the common speech conversations on a daily level. Most sentences have one verb, one subject and one object and no connections between simple sentences in the structure of the complex sentence. When exposed to such sentences, one may have the false assumption they understand a lot in general, but when confronted with complex sentence, they may not be able at all to understand the relations between subject, objects, simple sentences, etc... This is a huge problem between Bulgarian and Serbian as Bulgarian is analytic while Serbian is a synthetic language. Serbian and Slovene are both synthetic languages so this helps tremendously in understanding more complex constructions but one must keep in mind that the Slovene case system is not that similar to the Serbian one and is significantly more complex.
2) Every time we discuss (MI) between two languages we must observe objectively a wide range of lexical corpora in order to make final conclusion for the real average MI. I can not emphasize enough on how deceiving it may be to judge solely on fiction book texts, simple daily conversations, scientific texts, or movies scripts. Each language has an extremely tiny number of simple "key" words, usually up to 50, which in terms of frequency occurrences take more than ~90% if not more. These are the simplest but most important words. Each language has also tens or hundreds of thousands of words, which are encountered much more rarely as opposed to these simple words. But we need the simple "key" words in order to "unlock" the rest of the words as they are so important and so frequently used, that if we don't understand them we can't understand the rest of the information. I have observed an interesting phenomena where a pair of languages may have similar set of these few key words or exactly the opposite.
I feel like the Bulgarian/Serbian and Serbian/Slovene pairs have these type of corpora shared in an opposite way.
My observation is that the most frequent and important words are shared between Bulgarian and Serbian while the rest of the entire language is quite different. E.g., if a Bulgarian asks a Serbian for road directions, orders a meal in a restaurant, or even have a very simple daily conversation, chances are it may look like these languages are similar on 90%. On the other side, if a Bulgarian tries to listen to politic debates or to read a fiction book in Serbian, they may feel they understand at most 50% while in reality it might be even 30-40% due to the mistakes related to false friends (words which look or sound the same but mean something different). I think the relation between Slovene and Serbian related to these types of corpora is the opposite. E.g., a significant amount of the most common and important words seem to be different and completely strange especially to Serbians. But when reading fiction books, for example, my impression is that Slovene and Serbian share a significant part of their more rare and sophisticated vocabulary.
As already pointed out, Serbians should understand significantly less Slovene than vice versa. This is not only due to the historical and even current passive exposure to Serbo-Croatian in Slovenia coming from the diaspora, songs, TV etc... But also due the fact that many essential words in Slovene are unique to the language and do not share their roots with other Slavic languages, e.g. fant/punca (boy/girl). It also feels like in general Serbian is spoken clearly, "wide" and "open", while Slovene sounds more unclear due to the vowel reductions. Something like Czech and Slovak which share ~95% of their vocabulary but Slovak feels so much easier to understand, at least to me.
These are my impressions but please keep in mind that I am not a native speaker of any of these two languages so it is still hard to judge precisely.
I would like also to point out a huge disclaimer: all of the above observations refer to Serbian and official Slovene, which you will find in the books and you may hear for 30 minutes per day on the news. Otherwise it is not spoken anywhere and what is spoken on the streets is so different that without exaggeration it may not be more understandable than Chinese. When I first visited Slovenia I was already fluent in Serbian but didn't know any Slovene. I was understanding most of what was written on signs, etc... But when exposed to a 40 minute conversation in Vrhnika dialect I literally couldn't understand a single word of it.
