Just to get this out of the way, if you use discresion in your app download choices, you won't lose performance, you won't experience crashes, issues, etc. Just like your Mac or PC, don't download shady stuff, and don't download gimmicky bloatware. However, UNLIKE a Mac or PC, the amount of "Junk" even on Cydia, is limited. But, when your open, your open, there is no half way.
Now, if your wanting 4.2, for now your going to need a tethered jailbreak, also not really an issue. It only takes a minute to boot tethered, and your not going to run into a situation (most likely) where you NEED to restart your iPad. Many folks prefer an untethered 3.2 jailbreak and adding extensions like backgrounder to retain those 4.2 features. I, personally, run a tethered Redsn0w jailbreak for 4.2.1, with no problems.
Otherwise, why not jailbreak? The warranty issue? Just restore it in iTunes and you'll be fine, though there is a chance (albeit slight) that you get an issue that doesn't allow you to restore, so I understand that.
Failing that, unless you had the iPad 3G (which works with T-Mobile by the way, it comes factory unlocked to work with any GSM carrier, granted I don't know if T-Mobile offers service that works with the iPad, but the device is capable if T-Mobile offers it).
One thing I would look into, seriously, is an older WinMo phone, that runs at least Windows Mobile 5. There are tons of them on eBay, and they go for cheap a lot of times. Just make sure it's unlocked, and is GSM (has a SIM card). Then install an app called WMWiFi, and your golden. Pop the SIM into that phone, run that app, your good to go. Here's the thing, if T-Mobile has different plans for different phones you might have a problem, but what I have found with AT&T is, older Windows Mobile Phones (pre WP7) they don't always even consider smart phones (really! I had the basic unlimited web surfing plan for feature phones on my WM6 phone, touch screen, stylus and all!), so if T-Mobile is like that, you should be good to go.
There is also an App for older PocketPC versions as well, that works, but I don't remember the name of it. That would be useful if you found an old Cingular 8125 on eBay or something, which should work with T-Mobile. I used to tether via WiFi with my 8125 for a long time, running PocketPC 2003.
If you skipped to the bottom, bluetooth won't work without jail breaking, only WiFi tethering, so buy a MiWi, get a 3G iPad, or buy a phone that supports some form of WiFi tethering.
-John