update
so i have been using a new method for my LV Evo (no sleeve) and getting even better results. basically im doing a 1.5 minute warmup... yup thats right... 1.5 minutes. this almost completely eliminates any "primer hit".
i drop herb bits into my Evo until its full. then i pack that down lightly and drop in a little more. pack that down with a medium force. then a dome screen on top of that, pushed down contacting the top of the load. then i pop the mouthpiece on and hit the switch for 1.5 minutes. first hit tastes amazing and provides damn good vapor. i keep the switch engaged and hit it again. its almost like im smoking it. most of the time i just keep the switch on for the first 3 or 4 hits (dont spend a lot of time between hits). now you may be saying "wha wha WHAT???", but i assure you if you do this it will provide vapor like it never has. you can hit it slow or fast at this point it doesnt really matter. so now the heater is good to go and will provide tons of vapor. even though i know its not combusting sometimes i still have to unload it just to make sure and im amazed every time. so after 3 or 4 hits with the switch held on you will notice the device will start to get warm. at this point you can release the switch but i wouldnt let it cool too much between rips or youll be waiting longer than you want to get to that sweet spot again. obviously your cells charge wont last as long, but it is soooo worth it IMO. oh and go ahead an over pack it. the more the merrier (as long as it fits with the screen and doesnt block too much airflow). i just vaped a bowl that was exactly twice as much as i normally use and got huge rips.
im getting 1.5 bowls normally, per charge using this method. it is the only method i will be using from now on. it works like you imagine it should when allowed to heat up properly. IMO 45 secs will get you vapor, but let that heater keep going for another 15-45 secs and unleash the real LV Evo.
as for the SV Evo... i have no experience.
I use my Evo much as you do, with a direct mouthpiece-to-core connection, though not with quite as long a preheat. I have the sv Evo, and I think it heats up a little faster than the lv Evo. I've found that 30-45 seconds is plenty to get things working as you describe. For reference, my sv Evo core is glowing fully orange after 15 seconds from cold. Even after a 45 second preheat, I still take a priming puff or two to get the hot air moving through the load, before I start massive hit number one. I also don't let go of the switch until after the third or fourth consecutive hit, at which point the Evo is getting hot, and I want to give my lungs a break from the three or four monster hits. In the 30 second or one minute break, the evo doesn't have much of a chance to cool down, so a 10 second preheat gets me to the same place again, with another three or four big hits available. I find I get the most out of my loads by doing a three-or-four hit sequence, then a minute pause, then another three-or-four hit sequence. Though I can usually get another load out of the batteries if I don't let the Evo cool down too much, or at least a half a load even if I let the Evo cool down completely, I generally swap out batteries after each load. They take less time to recharge, and they'll last MUCH longer before needing replacement if you don't take them down to dead before replacing them. With 16 batteries, I can afford to treat them kindly and not run out before I can recharge them.
By the way, you should order another insulating sleeve, and cut the top portion off, and use it to cover the Evo core. It will save you from lots of inadvertent burns on the otherwise bare metal. If you put a note in with your order, you can ask Thermovape to not press the UFO into the insulating sleeve, but to send it loose with the sleeve, and you'll have a spare UFO to press into a mouthpiece if you want. You don't have to cut much off the top of the sleeve, just enough so that you can put the mouthpiece into the core without it hitting the sleeve. If you haven't already done this, and you can't quite picture what I'm talking about, let me know and I'll post a picture (or can search back in the thread and check out a picture that Jam posted of the modified sleeve).
I don't pack very hard, but I do tamp a little. When I tried packing it, the draw became excessive and it didn't work as well. I started out just using an extended length delrin mouthpiece, which worked great, but without a screen I did get bits of botanicals with every hit. It didn't bother me much, but at Tim's (Thermovape) suggestion, I pressed a UFO into the bottom of the mouthpiece, and it works GREAT and no bits in the mouth. The UFO adds 2mm or 3mm to how far the mouthpiece extends into the evo core, and it keeps the load pressed down as you inhale (as does your screen, from the way you describe it).
... I have however run the Evo 6v and the Evo 3.7v on a wooden ecig mods and I felt like the wood did an excellent job of isolating the heat to the Evo Heater Core instead of transferring the heat to the rest of the mod body.