Not sure what to tell you man, I've been up to 1790 but that was due to some games with Imok/rowe/suki , playing by myself I stay in the 1725-1775 range.

If I'm Blue or Pink and there's no obvs smurf on my team to go carry I'll play a carry ( mm/sw/mb most of time) and its most of time free mmr, if I'm in a game with 1850+ players I just go 100% support, up cour at start/stack/pull and keep up wards/counter.

Also I play mm/sw/mb because if my team are losing hard I can just farm 50 mins and still win.

Alucard got up to 2k doing nothing but solo. There's a vid on youtube of it as well.

It's very possible to get to a high rating solo. Back in beta I did 25-0 1700 psr solo pubbing. Also before TMM was out and SMM was the only thing available I think I got the highest rating in the world (like 1794) with 70% winrate or so playing it solo. And don't tell me there's alot more bads now so its harder. There were plenty of bads when I played that tried to roop me every second game just to ruin my winrate / record. I just adjusted to it and made the most out of the situation. Also, more bads means the enemy team is likely to have a few baddies too. Just kill them a few times and make them rage @ their team and the games pretty much sealed. A few tips to keep in mind when you're solo pubbing are:

1) Unless you know how to play amazing pub support while still getting enough farm and levels to have a presence in the game, I suggest you solo mid as semi-carry every game. It has way more impact on the game than some pure support hero and allows you to dominate single-handedly.
2) Never EVER give up anything for your teammates. When I played I would take every rune, every kill, and I would never give up mid lane for anyone. The reason being that I knew I could rely on myself but not on them. If some kid tried to come mid lane I would tell him to fuck off once then I just wouldn't say anything. After I got 8/8 CS the first wave and he was stuck with nothing he normally ended up leaving.
3) Use voice chat. Seriously fucking YELL are your team when you're about to gank. The WORST thing you can do as a mid solo semi-carry is attempt a gank and get nothing out of it. Tell your team to stun like 500 times over voice chat if you have to. Normally they'll listen to voice more than to text.
4) Keep your cool. You can BM/rage all you want as long as it doesn't affect your gameplay. If you find yourself typing or getting angry and missing easy creep kills or just playing badly in general, stop raging and focus on your game.
5) FARM. I can't stress this enough. So many times I see kids dominating their mid lane then by 10 minutes they have like 40 CK max. WHAT THE FUCK IS THAT SHIT? If you're missing 1/2 CK every wave despite doing well in your lane you're not devoting yourself to the game, and you deserve to lose. You want at MINIMUM 5 ck / minute throughout the game. I've done like 6 ck/min before while also ganking the fuck out of the other lanes. I've also done 8ck/min before when I just sat mid and farmed and Soulstealer. DO NOT GIVE UP CK. 5 creep kills roughly = 1 hero kill in terms of gold you get. Sure a hero kill is better since it sets the enemy behind, but do you really think its possible to get 1 kill a minute every minute? FARM.
6) Be aggressive and take risks. At the start of the game you normally have 1 set of tangoes + 1 healing potion. That's enough to restore your hp like 2-3 times over. Get aggressive against your opponent mid. Harass the fuck out of him. Who cares if you take some damage, by the time you get your bottle if you still have those initial regen items they're basically wasted. Focus on CK more than anything of course, but always harass your opponent in between creep kills. Go past the wave onto his ramp at early levels if you're dominating the lane. Press every advantage you have.

I could go on forever about this topic but those tips should get you started. GL

Took these tips. Went Nomad mid. Raped the fuck out of 3 games in a row. Ty rate.