Email for issue tracking hasn't scaled for me past the 200 customer mark, after that it was Highrise then FatFree CRM all the way. Way too useful to be considered an "extra".
In general, I'd say support bandwidth is dedicated more to a "moving window" of how many new customers you have, rather than total customer base. Most customers tend to be pretty low-touch after the first month, where they get most of their questions answered.
(btw, just posted a more detailed reply to your comment on the article itself)