The question was whether the number of rewrite rules in .htaccess would have a significant impact on performance.

With the following set of rewrite rules:

Options +FollowSymLinks -Indexes
<IfModule mod_php5.c>
        php_value magic_quotes_gpc 0
        php_value magic_quotes_runtime 0
        php_value allow_call_time_pass_reference 0
</IfModule>
DirectoryIndex index.php
<IfModule mod_rewrite.c>
        RewriteEngine On
        RewriteRule ^cms/index\.php/plain_admin$ http://domain.com/admin [R=3
01,L]
        RewriteRule ^cms/index\.php/plain(.*) $1 [R=301,L]
        RewriteRule ^media.* - [L]
        RewriteRule ^custom.* - [L]
        RewriteRule content/treemenu/? /index_treemenu.php [L]
        RewriteRule ^var/[^/]+/cache/public/.* - [L]
        RewriteRule ^var/storage/.* - [L]
        RewriteRule ^var/[^/]+/storage/.* - [L]
        RewriteRule ^var/cache/texttoimage/.* - [L]
        RewriteRule ^var/[^/]+/cache/texttoimage/.* - [L]
        RewriteRule ^design/[^/]+/(stylesheets|images|javascript)/.* - [L]
        RewriteRule ^share/icons/.* - [L]
        RewriteRule ^extension/[^/]+/design/[^/]+/(stylesheets|images|javascripts?)/.* - [L]
        RewriteRule ^packages/styles/.+/(stylesheets|images|javascript)/[^/]+/.* - [L]
        RewriteRule ^packages/styles/.+/thumbnail/.* - [L]
        RewriteRule ^/favicon\.ico - [L]
        RewriteRule ^/robots\.txt - [L]
        # Uncomment the following lines when using popup style debug.
        # RewriteRule ^/var/cache/debug\.html.* - [L]
        # RewriteRule ^/var/[^/]+/cache/debug\.html.* - [L]

        RewriteCond %{HTTP_HOST} ^webdav\..*
        RewriteRule ^(.*) /webdav.php [L]

        RewriteRule .* index.php
</IfModule>

The results were:

Received: t=1267660375832889 Duration: D=273652
Received: t=1267660761803171 Duration: D=297682
Received: t=1267660779066518 Duration: D=272959
Received: t=1267660804103095 Duration: D=292675
Received: t=1267660821137470 Duration: D=256268

This set of rewrite rules:

Options +FollowSymLinks -Indexes
<IfModule mod_php5.c>
        php_value magic_quotes_gpc 0
        php_value magic_quotes_runtime 0
        php_value allow_call_time_pass_reference 0
</IfModule>
DirectoryIndex index.php
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^media.* - [L]
RewriteRule ^custom.* - [L]
RewriteRule !(\.(gif|jpe?g?|png|css|swf|mp4|js|html?)|var(.+)storage.pdf(.+)\.pdf)$ index.php
</IfModule>

Yielded these results:

Received: t=1267660624947466 Duration: D=279933
Received: t=1267660660237894 Duration: D=289207
Received: t=1267660681578539 Duration: D=327192
Received: t=1267660701894745 Duration: D=254252
Received: t=1267660719493366 Duration: D=262131

This was a very informal test, just to see if there were any glaring differences, as you can see, the results were reasonably close.