Spamassassin spamd basic configuration

I am running suse openexchange which uses spamassassin spamd version 2.6.3.
spamd is started with the options '-d -c -a -L -u nobody'.

spamassassic configuration file contains :-
# rewrite the Subject: line with ****SPAM**** .* if set to 1 (default=1)
rewrite_subject 0
# report briefly, recommended for report_header==1 (default=0)
use_terse_report 1

Currently spamassassin is only detecting about 30% of spam which is quite poor.
Openexchange does have some strange defaults in some places and I am questioning the use of the '-L' parameter which disables network based checks.
I have looked in /usr/share/smapassassin at the 20_dnsbl_tests.cf and 50_scores.cf files.
1) Spamassassin checks some RBL lists and the -L parameter will effectivly disable this?
2) I would imaging that enabling these checks will make quite a significant difference?

We have an 1024/256 ADSL line and get approx 1500 emails a day (just over 1 per minute).
The mail server runs its own DNS with no forwarders defined.
3) Will there be a lot of additional load on the ADSL with these additinal checks which I believe are normally DNS based?