I really wouldn't worry about it unless you're flying in from a regional airport that is popular for Brazilians at Carnaval e.g. Salvador or Porto Seguro.

GRU primarily serves international flights. I flew out of there just after NYE and last July on the first weekend of the Brazilian school holidays both of which are busier than Carnaval in terms of getting to the US. Assuming you have elite status for check-in the only potential bottleneck is the Policia Federal check. This is very well organised at GRU (as is security) and even during these busy periods only took 5-10 mins to get thru i.e. much quicker than GIG at regular times of the year.