That is a bit speculatice and complicated as well. "Polydactyly" is a nice catch-all for "hands not normal" with regard to number of digits. But the actual manifestation of this overall rare condition can be of quite the variation in both forms and causes.
Whether it's a genetic/epigenetic component a teratogenic influence on pregnancy or a comorbidity in a number of syndromes…
Two papers to highlight the problems with polydactyly or with polydactyly 'removed' show that what is most often seen as an aberration and at least cosmetic problem might not be as much a functional problem as a surgically 'corrected' extremity might be.
There are three factors that determine the impairment of a hand with central polydactyly. First, hypoplastic joints and soft tissue changes in the polydactylous and adjacent digits predispose to joint contracture and angular deformities. Second, delta phalanges predispose to progressive angular deformities. Third, joint contracture and rotatory deformities occur after extensive separation of complex osseous syndactyly, which may be further complicated by a disturbance of differentiation of tendons between affected digits (Fig. 3). Our results show that the creation of a five-digit hand usually leads to poor functional results. Despite this, all parents were satisfied with the five-digit hand for cultural and religious reasons. For the reconstruction of a functional hand, the most hypoplastic ray should be removed and a functional four-digit hand should be the object of the reconstruction surgery.
–– Koichi Tada et al.: "Central polydactyly – A review of 12 cases and their surgical treatment", The Journal of Hand Surgery, Vol 7, No.5 September 1982.
And an 'uncorrected' 24 digit owner from India being a wrestler in his youth:
Local examination showed extra digits in all 4 limits So total, he had 12 fingers in his upper limbs (6 in each ) and 12 toes in his lower limbs (6 in each). All the toes and digits were mobile except in extra toe where there was limitation of movements and were almost same calibre of the remaining fingers /toes. No other anomalies were observed.
A 68 year sold male from Kumbalagud, Mysore road, Bangalore was found to have a total of 24 digits in both upper and lower limbs bilaterally. He had no other anomalies with any signs and symptoms. He was father of 4 children where second son had extra limb in his right upper limb who had got married to his own relative but exact relation is not known. An elderly person with well built and well nourished body was very tall nearly 6feet and 7 inches. He had no systemic diseases. He was a wrestler in his younger days. There was no cyanosis, clubbing jaundice and lymphadenopathy. All his systems were within normal limits.
–– Arun Kumar S. Bilodi & M.R. Gangadhar: "Supernumary of Digits and Toes: A Case Report", IJCRR, Vol 05 issue 06, 2013. (PDF)
As the motto "if it works, don't fix it" seems to be void when cosmetics and cultural concepts enter the arena, we have to keep in mind:
Ulnar polydactylies present with a wide variation in the size and appearance of the affected finger, ranging from small cutaneous appendages to fully formed additional fingers. Since ulnar polydactyly is rarely associated with functional disabilities, psychological and cosmetic reasons are the main rationale for surgical treatment. The aim of this study was to present the appearance, treatment regimen, and outcome of a series of children treated with ulnar polydactylies.
–– G Singer: "Ulnar polydactyly - an analysis of appearance and postoperative outcome", J Pediatr Surg. 2014 Mar;49(3):474-6. doi: 10.1016/j.jpedsurg.2013.06.029.
In summary I'd conclude that comparatively rare as 'polydactylies' are 'fully formed and fully functional polydactyly' (FFFFP) seems to much rarer still. Whether surgical alterations are indeed a functional improvement seems not always to be the case. But it seems rarest to encounter any FFFFP patient which had his/her extra-limbs tolerated and trained, for example for a complicated instrument like a piano, to get at any observation that would be worth generalising. But as long as the mechanics of the digits are indeed independently functional, I am quite convinced the brain would be adaptable to that situation.