Extrapolating performance expectation

Can one extrapolate future performance expectations for ever-growing tables
from a given (non-trivial) data set, and if so with what curve? Corollary:
what would one expect a performance curve to approximate in terms of query
execution time v. number of data rows (hardware, load staying constant).

I have user and group information on system usage. I would like to be able
to do year-to-date counts per user given a single group id but in the data
for one business quarter the query is taking in between 10 and 60+ seconds
depending on both on the size of the group and the group's total usage.
Groups typically have 10-100 users and consume 20K - 80K records in a 9M
record data set. Group id column is indexed, but it is not the primary
index. (Sad note: two pseudo groups account for 50 percent of the total
records IIRC (and will never be used for the usage-by-group query below)

This is a single table query:

select user_id, element_type, count(*)
from dataset
where group_id = N
group by user_id, element_type
order by user_id, element_type

Is this the sort of situation which might benefit from increasing the number
of histogram bins (alter table alter column statistics (N>10))?

Any and all pointers appreciated,