AFAIK the Twilio limit (regardless of shortcode or full sending #) is 100 concurrent inbound requests to the API per API key. You'll never have to worry about this w/a Marketo webhook because Mkto will only parallelize ~8 concurrent requests.
The shortcode difference comes into play after the webhook stage is complete and relates to how fast Twilio pulls messages off its internal queue and actually sends outbound via SMS. At that stage, a shortcode is vastly better because they'll send the outbound SMS at 100 messages per second if you have a shortcode but only 1 per second if you do not -- in other words, you get 100x faster delivery with a shortcode.
So for high-volume, time-sensitive sends, the key is to get things into Twilio faster than a webhook's inherent speed limitation. In this case it's better to use the Notify service, which can send all your messages with a single API call. The way to use Notify in this case is to run everyone who qualifies through a webhook that only adds them to a Notify "segment"; you can run this webhook the day before the send, for example, or whenever you know someone should be added to the future send. Then on the day of the send you only call the webhook once, telling it to notify everyone who's been added. (You can send the webhook in the context of a special lead you dedicate for this purpose, so you can use the Marketo batch scheduler as you usually do.)
