I would be wary of things you read – e.g. there’s a common myth that “having lots of plugins” is a bad thing, whereas of course what matters is whether they’re well written or not. The time to load a large number of very well written plugins is neglible, whereas a line of bad code in one can easily screw things up for everything else.
There might be a case to split a plugin if it makes sense structurally, and you are likely to be able to reuse the individual plugins elsewhere, but concentrate on improving the quality of your own code first:
- are you using classes (correctly)?
- are any of your classes or methods becoming unmanageable (e.g. cyclomatic complexity) – use a tool like PHP Mess Detector
- are you making common mistakes? try PHP Code Sniffer or JS Hint
- are you using the database correctly/efficiently? e.g. don’t use query_posts ever
- are you following the coding standards
- is your code well documented? PHPDoc
Make iterative improvements.
Find a good IDE.
Refactor often.