Compared with adding multiple rows to a table to simulate one local variable? I expect readability and performance will both be greatly improved, but... it's hard to offer a good recommendation without knowing what sort of calculation you're trying to perform.
I've found performance pretty decent when using hstore with indexes. You should read about what's coming in hstore 2, also.
Do you have a good description of hstore 2? A google search found a few references that there is an hstore 2, and that it might be related to PG 9.4, and that it might allow the values to be non-string, and it might allow nesting... nothing very clear.