Surefire product holds its value, and though every modder believes they will increase the value with a mod, the reality with Surefire pieces is that modding an OEM Surefire head will be a wash (value will not increase), and more likely, it will half the resale value of the piece (even if it is far far better), and at worst, it will be immeasurably less reliable.
Up until very recently, there were cheap aftermarket E series LED conversion heads available on eBay for $20 or less, but just looking now, I only see Surefire OEM and Lumens Factory upgrades, which is quality and performance for which you must pay. But I was going to recommend grabbing a cheap knockoff E series LED head, then modifying that to your heart's content (or just send to Vinh for extreme LED services, or High CRI LED swap, copper bonding, etc.), while leaving your stock Surefire pieces and their warranty intact.
Sometimes you can find loose E Series Elite incan heads, and Lumens Factory will be eventually releasing a version of the E Series incan head. You could get an incan head, a Tad Customs socket and lamps for running T. Customs durable and inexpensive xenon bipin lamps with primary or secondary cells. With fivemega's FM1794 lamps, and adjusting an Elite bezel to allow room for the longer lamp, you can achieve over 1000lm of incan with IMR cells, which would be a major upgrade in brightness and throw for E2L Outdoorsman (though runtime will be reduced drastically).
Overready made the WASP v5, some great triple LED heads that sometimes reappear in WTS areas on these forums. Sunwayman' v11r head is E Series compatible, with infinintely variable brightness.
However...
A VME head with a Malkoff drop-in is a nice option.
is probably... the quickest, slickest and most reliable option (among a vast array of stuff you can do instead), will likely bring the most satisfaction for the longest period, and increase the value of the system indefinitely for at least what you paid (granting a fuzzy margin depending on how fast you wanted to sell).