seems the feeling is that if you have multiple full feeds and need to loadshare, you really don't want (in most cases) ispa=500mbps + ispb=500mbps.
you really want destinationA to be reached across the 'best path' (best ... in some form, distance? packetdrop%? jitter? cost?) you'll most likely have to tweak things in order to achieve what you want since only distance is really used in the stock bgp calculation (distance by as-hops, presuming you don't listen to closely to med from your providers)
Yes, but performance from your network to $destination_AS via $ISPx can be variable and how do you know when it changes before someone starts complaining? There are traditionally two pieces involved with optimization. 1) "Cost" (Commitment/oversubscribe management and monitoring) 2) "Performance" Usually "cost control" is #1 so systems like that are configured so that as long as the traffic isn't breaking your commits or filling your pipes they will then optimize X number of your top prefixes for performance (based on what the system can see). The performance aspect is generally just sending basic probes in all directions towards a destination host and seeing which ones reply the fastest. Although obviously this only impacts traffic outbound from your AS. -Drew