I have read in several places that people use big, fancy, complicated, and little known words (such as Brobdingnagian) to give the impression that they are knowledgeable, smart, and professional. Does that work?
Although this doesn't answer your question directly, I think it does a good job of answering indirectly.
Research shows a strong correlation between vocabulary and general intelligence. So does using big words make you appear smart, maybe, but having a high level of vocabulary (and being able to use it) would indicate that you actually are smarter.
The key to the sentence above is actually being able to use the words correctly. Just going out and learning a bunch of words is not going to immediately make you smarter, but having a strong grasp of the language and a wide vocabulary indicates you are smarter.
Your question has a large subjective part to it, because if I am really clever and use lots of clever words I may be smarter than someone with a low I.Q, but they may just think I'm a dick. This means that your question answered in that way can't be answered objectively.
Now on to some examples:
Analysis indicated strong correlations between the two measures, particularly between the CREVT General Vocabulary and WISC-III Verbal IQ (r = .80), WISC-III Verbal Comprehension Index (r =.83), and the Vocabulary subtest (r =.76). These results held across the grades. (Smith, Smith, Taylor, & Hobby, 2005).
Acquisition of word meanings, or vocabulary, reflects general mental ability (psychometric g) more than than do most abilities measured in test batteries. Among diverse subtests, vocabulary is especially high on indices of genetic influences. Behavioral and Brain Sciences (2001), 24: 1109-1110 Copyright © 2001 Cambridge University Press
This page provides about 10 examples with unlinked references.
Shows high positive correlation between JOCRF vocabulary score and SAT-verbal. Bowker, R. (1976)
“English vocabulary level has been shown to be strongly related to educational success. In addition, it is related to the level of occupation attained. It is highly correlated with measures of reading ability and intelligence” Bowker, R. (1981)
If you're in an interview with an expert, it's better to read books on subjects they want you to know. If you're not talking to a expert, they'll be baffled by unfamiliar vocabulary and unable to determine what you're talking about. In either situation, especially with a expert, mistakes in how you use advanced vocab will cost you.
I.E., it's more important to use vocabulary that excites and motivates people (read: stay positive) than to fake knowledge and experience that you don't have.
The only motivator I can think of to use big words would be to drive off competition, and only a small amount at that.
Your question can either be answered by yes and no, distinctively separated by your definition of "professional". Would I consider my doctor using big words, professional? I don't think so. At the same time can it be convenient, if I am with peers, to be able to share thoughts using big words as abbreviations of complex concepts.
Goethe with his "in der Beschränkung zeigt sich erst der Meister" (in limitation one recognises the master.) and Bonnot de Condillac stating that "every science requires a special language because every science has its own ideas."