Very true..
Work with the network operators on each side of the link to determine the speed/load. For the most part if they really want your business, they will be able to provide something.
The main reason link speed maybe important to me would serialization delay on the circuit. OC-768 should be much lower latency than a T1...unless your are at the end of the queue 
Latency is probably be your primary concern for large TCP transfers anyway.
-C
Work with the network operators on each side of the link to determine
the speed/load. For the most part if they really want your business,
they will be able to provide something.
Actually, many larger peering relationships come with contracts which
explicitly forbid them from telling you any details about link capacity or
utilization, or in some cases even acknowledging that peering exists. They
might tell you anyways though.
For the most part, it is hope for descriptive DNS or bust. For the most
part, DNS is not descriptive (especially on peering links).
The main reason link speed maybe important to me would serialization
delay on the circuit. OC-768 should be much lower latency than a
T1...unless your are at the end of the queue
Once you get past 10Mbps, serialization delay is measured in fractions of
a millisecond. This has absolutely no impact on TCP performance, compared
to speed of light delays (like from oh say a 50ft patch cable).
Latency is probably be your primary concern for large TCP transfers
anyway.
I think that you are very confused about how TCP works.
> �4 t3-1-2-0.ar2.SEA1.gblx.net (64.211.206.113)� 20.436 ms� 18.309 ms�
> 17.605 ms�� <------------DS3
> �5� so1-0-0-2488M.ar4.SEA1.gblx.net (67.17.71.210)� 17.607 ms� 16.982
> ms� 16.971 ms� <-----OC-48
> �6� p3-3.IR1.Seattle-WA.us.xo.net (206.111.7.5)� 17.864 ms� 19.491 ms�
> 17.181 ms
Name: p3-3.IR1.Seattle-WA.us.xo.net
Address: 206.111.7.5
Name: 206.111.7.6.ptr.us.xo.net
Address: 206.111.7.6
Sometimes you can find descriptive DNS on the other side of the PTR, and
sometimes you find missing data. In this case, there is no speed
description at all. The p3-3 interface indicates that this is a PoS
interface, probably Cisco, and the IR designation on XO indicates a
peering router. Educated hunch based on what the traffic levels between
3549 and 2828 in Seattle probably are... OC3?
Of course individual links are likely to have little difference .. it'll come
down to a combination of the smallest link between source and destination and
the end to end latency.
Steve