Antony, it is not having too much data that contributes to loss of situation awareness; it is more likely to be poor mental discipline in focussing attention on the appropriate data, i.e. filtering out the unnecessary data (at that time).
Your focus of attention is the dominant factor in determining what is ‘seen’, and thus what is understood. This, together with a comparison of previous knowledge (mental model), provides a ‘true’ awareness of the current situation. Therefore you have to know which data is important in each scenario, and where to find it, i.e. in level flight you scan altitude frequently, but during climb and decent less frequently.
The tasks above are essential elements of an instrument scan; this determines what ‘is seen’ how often it is reviewed and the sequence that enables you to build a mental picture of the situation. The concept of an instrument scan can be applied to most aspects of flying; i.e
Plane, Path, People. An ‘old’ flight deck might appear to have ‘a lot’ of information, but provided there is a logical layout, well marked instruments, and clear indications, then these old aircraft provide a good basis for an instrument scan.
Conversely, some modern aircraft with EFIS have a similar amount of information, but due to a ‘cluttered’, compact format the information is not easily ‘seen’, e.g. EFIS might show the label ‘DME’ next to a DME beacon range and ‘DIST’ for LNAV range to a waypoint; given appropriate circumstances enabling human error, then these could be interchanged resulting in a loss of situation awareness.