Man, I wish I had gotten started on Teemo earlier than this week... Now I will have to purchase him to try out some of your tips. Unfortunately, I'm still trying to finish my Ashe runes, purchase Twitch, Miss Fortune, and possibly Tristana, and then I'll get Teemo and start working on the Magic Pen runes for him.
On the whole movement speed thing, Boots of Swiftness do probably offer a bit of extra speed. However, the amount is probably negligible given the fact that one item that can improve your movement speed to ALMOST the same final movement speed.
The perk to ignoring Swiftness is that the other boots actually have combat benefits. I'll use Ashe as an example, since she's my main and I know more about her.
According to Leaguecraft, her base movement speed is 300. For my example, I'm going to compare the benefit of Berserker's Boots & Zeal to getting Boots of Swiftness & Zeal.
Zerker's grants 70 additional movement, and Zeal increases movement by 8%. So, 300+70=370. 370*1.08=399.6. Final movement speed is 399.6, but you get an extra 25% to attack speed.
Swiftness grants 90 additional movement, and Zeal increases movement by 8%. So, 300+90=390. 390*1.08=421.2. However, the 6.2 over 415 is only 4.96 due to the 20% reduction, so, the final movement speed using Swiftness is actually 419.96
As you can see, you only have to sacrifice 20.3 movement for an additional 25% attack speed. Or you might decide to get Tabi or Merc's, but either way, the sacrifice of a measly 20 movement speed is well worth it, in my opinion. We're only talking about a 5% increase in movement speed anyway, so I don't think it's really going to get us anywhere.
How this fits in with Teemo, I'm not entirely sure. I would think that since the return on investment into movement speed decreases dramatically due to the soft cap of 415 movement speed, it's just not worth sacrificing something like attack speed for that extra 20 movement speed. Especially considering Teemo already has an ability that enhances movement speed.