I currently have EL-34s in my Mark V. The harsh highs on the V with the stock 6L6s were part of the reason, and since my other Mesas have 6L6s in them, it's good to have one with EL-34s too.
Aside from adding meaty midrange grind and crunch to your sound, one of the big benefits is indeed that they make highs a bit more tolerable, less "icepick-in-your-ear". I could never use the bright switch (on channel 3) for band playing when I had 6L6s in my V, or the cold, piercing highs would have made my ears bleed!
I wouldn't say you necessarily lose that much lows if you use tubes like SED =C= EL-34s - the lows may be a little less prominent because they are masked by the strong midrange punch, but you can still get them. You may need to turn up the bass pot on your V to achieve that. And with EL-34s, you actually can do that - without getting the mush and flub your amp is prone to give you if you turn the bass pot past 9 o'clock with 6L6s.
Channel by channel, this is how I feel SED EL-34s compare to the stock Mesa 6L6s:
Channel 1: While the smoother, prettier, shimmering highs on the EL-34s are certainly an improvement over the cold, thin, tinkling 6L6 highs, the clean sounds definitely lose some body and bloom. To compensate, you have to scoop your sound slightly and especially turn up the bass pot. But that will only get you so far - this channel simply works better with the 6L6s.
If you mainly stay on channel 1, keep the 6L6s, and use your EQ to tame the piercing highs. Simple as that.
Channel 2: If you haven't heard this on EL-34s, you haven't heard what your channel 2 is capable of! Period.
No, EL-34s will not turn your Mark V into a Marshall...but they will give your Edge and Crunch modes a distinctly British accent. Those two modes make a lot more sense that way. Add an overdrive to an EL34-powered Edge or Crunch mode, and you've got instant late 1970s/80s heavy rock sound!
As for the Mark I mode...wow. Just wow. With EL-34s, it's unlike any other sound I've ever heard from any Mesa amp. It's like an ultra-hotrodded Marshall. Except that it doesn't sound like a Marshall at all. No, it sounds more like what a Marshall wants to sound like when it grows up. And you can even use the Thick mode and turn up the bass knob in your rhythm playing and sound even bigger (not flubby and tubby like with 6L6s).
With EL-34s, the Mark I mode is easily my favorite distorted sound on the whole amp - and yes, that's counting the channel 3 modes. The Mark I mode is so much bigger and badder. With EL-34s.
Channel 3: This is mostly a matter of taste. Some people will prefer EL-34s here, some will prefer 6L6s. Obviously the EL-34s will give you more midrange grind and crunch, while 6L6s will make your channel 3 modes sound more massive and more modern. Roughly, if your ideal metal sound is from the 1980s or early 1990s, you will probably like the EL-34s better on channel 3. If your ideal metal sounds are from the Rectifier era, you will probably like the 6L6s better here. I went with EL-34s but that's at least partly because I have a couple other, 6L6-loaded Mesas in my arsenal.
Also, with EL-34s, the lack of piercing ice-pick highs is a big advantage on channel 3 - I can actually use the Bright switch now.