Yes, it would apply to ADSL2, provided your line can achieve the actual bitrates required. I don't know however, whether or not Slingshot customers would get the same level of performance... I'm not saying they won't - I'm saying I know our network, where the data streams come from, and the loadings of the servers and links involved... I know none of those details for Slingshot.
And no, a simple speedtest won't give you anything like the whole story I'm afraid.
Cheers - N