a massive guitar, regardless of the wood type, generally gives off more lower-end through the amp. what we often deem as 'thick' is actually more lower frequency in the mix. the wood type only serves to further the frequency discrimination. eg: ash brighter than basswood
my Ibanez S540 is mahogany but the 'thickness' when plugged in is rather lacking (note: the S-series' body make is super thin). the reason why i have Duncan's Invader in it (bridge)- to offset the lack of bass.
do note that the EMG (or any other active unit for that matter), at neutral settings, does not inherently give off that 'thick' voicing. in fact, it sounds bright with loads of bite. how does Metallica achieve that thickness? Answ: EQ. metal maniacs (myself included) enjoy the intense crunch coming from an EMG & its counterparts because they contain that source-induced boost which only an active unit can achieve well.
if you think your tone is gonna benefit from a pickup swap- by all means, go ahead. otherwise, it's more sensible to hear your guitar through a 12" speaker equipped amp & assess your needs accordingly.