I have tried up to 25 degrees advance in my '82 380 SL, but that
requires premium gas to prevent knock. There is a significant increase
in power, but I do not run with that much advance and definitely do NOT
recommend it. You need a timing light and be careful, as there are a
bunch of thing moving around pretty fast near where you will be
working. In addition to the spring issue discussed above, cars this
old usually require the vacuum lines to be replaced and often the
vacuum advance diaphram leaks. Fix all of those problems before
playing with timing. A really long handled hex wrench is a good idea
to keep hands out of harms way.
If you are not worried about long term durability, keep in mind that it
is possible to trash your engine in a few seconds with too much
advance. Watch your engine temperature, check your mixture, and
consider low speed drivability/starting issues.