Executive summary of this post:

[EDIT: I run these tests with WiFi DISABLED, Bluetooth on but not connected, and Network mode set to 3G. Tests are performed locally on the N900 (not via tethering to a PC) and are run in the default built-in browser. Running stock kernel, PR1.3, no other network or relevant CPU load during tests.]

15Mbps!
- My HSPA+ download rates are now consistent at ~15Mbps (t-mobile usa, rates calculated from single 50MB downloads to qualify as 'consistent' in my view).

10 Mbps max?
- I thought the N900 would only scale up to 10Mbps

What is the typical experience?
- Anyone else seeing this? What are the fastest rates observed on the N900 so far?

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Now the details (including an important note on test methodology):

I've been running speed tests from my device to see if my area is covered by the latest HSPA+ improvements (tmobile usa is rolling out 21Mbps service).

In short, yes. I now get download rates anywhere from 10Mbps to 16.8Mbps for single large transfers (small transfers measure too fast to be realistic and are therefore considered faulty: one test measured 88Mbps on a 1MB transfer, clearly noise in the calculation).

I used to use these two sites for speed tests:
http://dslreports.com/mspeed
http://speedtest.net
http://speakeasy.net/speedtest

Lately, however, I have found these speedtests insufficient. In all cases, the site would begin with a ping test, followed by a download test of unspecified size. The download would start slow (0.25 seconds at ~1Mbps) before jumping to some higher rate. In these cases, it seems the download SIZE was too small (perhaps dynamically adjusted to allow a 1Mbps download to take several seconds).

In short: the above sites would arbitrarily measure my connection rate either very slow or somewhat fast, and do so in a split second (overall, about 2x the latency; visually approximated) which makes these tests useless in my opinion.

Seeking other sites I found these two:
http://www.internetfrog.com/mypc/speedtest/
http://testmy.net/download-speed-test.php

The first uses, again, an unspecified download size. However, it seems to be adequate for testing ~15Mbps connections. Results from both sites agree on overall performance (within reason).

I prefer the second site, as it allows you to specify the exact transfer size. I used 50MB transfers for my tests.

Connected to T-Mobile USA in Western MA (Near Amherst MA 01002) I am able to repeatedly test my download rate using 50MB transfers and observe 10-16.8Mbps rates; typically centered around the 15Mbps result. Occasionally I will see the rate drop to ~2Mbps (about one test in 20) with the vast majority of test results are significantly greater than 10Mbps.

Greater than 10Mbps. I was under the impression that the N900 was only spec'd to scale up to 10Mbps. How am I getting transfer rates so much faster than this? Was it part of a firmware upgrade? Or was the original spec wrong (or did I misread it)?

Anyone else out there able to breach 10Mbps transfers with their N900s?