new chain and sprockets

okay....
going from a 525 --> 520 and also the OEM 15/40 F/R --> 14/42 #teeth of new sprockets... going alloy. also going thru chain gang (got all this info so far from the forums archives
) this will equate to a final drive ratio of 3.00 (instead of 2.67) supposedly an increase of about 2-8% RWhP????
whatever.... i'm mainly interested in LONG_LIFE and lightness of parts, okay, yeah, and the increase in power associated with the benefits of less reciprocating mass.... etc...etc...
BUT
will my speedo be more or less out (they reckon the things r already out anyway, this conversion may bring it back down??)
ALSO
just wondering what others would do in my case.... I NEED the whole new drive line set, so stay stock or just go MOD crazy while i NEED the whole bang lot anyway.....??????
going from a 525 --> 520 and also the OEM 15/40 F/R --> 14/42 #teeth of new sprockets... going alloy. also going thru chain gang (got all this info so far from the forums archives


BUT
will my speedo be more or less out (they reckon the things r already out anyway, this conversion may bring it back down??)
ALSO
just wondering what others would do in my case.... I NEED the whole new drive line set, so stay stock or just go MOD crazy while i NEED the whole bang lot anyway.....??????