Selecionando um subconjunto de linhas que excedem uma porcentagem dos valores totais

SQL Server 2012+ apenas

Você pode usar SUM em janelas :

WITH cte AS
(
   SELECT *,
          1.0 * Revenue/SUM(Revenue) OVER(PARTITION BY [User]) AS percentile,
          1.0 * SUM(Revenue) OVER(PARTITION BY [User] ORDER BY [Revenue] DESC)
                /SUM(Revenue) OVER(PARTITION BY [User]) AS running_percentile
   FROM tab
)
SELECT *
FROM cte 
WHERE running_percentile <= 0.8;

LiveDemo

SQL Server 2008:

WITH cte AS
(
    SELECT *, ROW_NUMBER() OVER(PARTITION BY [User] ORDER BY Revenue DESC) AS rn
    FROM t    
), cte2 AS
(
    SELECT c.Customer, c.[User], c.[Revenue]
           ,percentile         = 1.0 * Revenue / NULLIF(c3.s,0)
           ,running_percentile = 1.0 * c2.s    / NULLIF(c3.s,0)
    FROM cte c
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM cte c2
          WHERE c.[User] = c2.[User]
            AND c2.rn <= c.rn) c2
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM cte c2
          WHERE c.[User] = c2.[User]) AS c3
) 
SELECT *
FROM cte2
WHERE running_percentile <= 0.8;

LiveDemo2

Resultado:

╔══════════╦═══════╦═════════╦════════════════╦════════════════════╗
║ Customer ║ User  ║ Revenue ║   percentile   ║ running_percentile ║
╠══════════╬═══════╬═════════╬════════════════╬════════════════════╣
║        2 ║ James ║     750 ║ 0,384615384615 ║ 0,384615384615     ║
║        1 ║ James ║     500 ║ 0,256410256410 ║ 0,641025641025     ║
║        7 ║ Sarah ║     600 ║ 0,444444444444 ║ 0,444444444444     ║
╚══════════╩═══════╩═════════╩════════════════╩════════════════════╝

EDIÇÃO 2:

WITH cte AS
(
    SELECT *, ROW_NUMBER() OVER(PARTITION BY [User] ORDER BY Revenue DESC) AS rn
    FROM t    
), cte2 AS
(
    SELECT c.Customer, c.[User], c.[Revenue]
           ,percentile         = 1.0 * Revenue / NULLIF(c3.s,0)
           ,running_percentile = 1.0 * c2.s    / NULLIF(c3.s,0)
    FROM cte c
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM cte c2
          WHERE c.[User] = c2.[User]
            AND c2.rn <= c.rn) c2
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM cte c2
          WHERE c.[User] = c2.[User]) AS c3
) 
SELECT a.*
FROM cte2 a
CROSS APPLY (SELECT MIN(running_percentile) AS rp
             FROM cte2
             WHERE running_percentile >= 0.8
               AND cte2.[User] = a.[User]) AS s
WHERE a.running_percentile <= s.rp;

LiveDemo3

Resultado:

╔══════════╦═══════╦═════════╦════════════════╦════════════════════╗
║ Customer ║ User  ║ Revenue ║   percentile   ║ running_percentile ║
╠══════════╬═══════╬═════════╬════════════════╬════════════════════╣
║        2 ║ James ║     750 ║ 0,384615384615 ║ 0,384615384615     ║
║        1 ║ James ║     500 ║ 0,256410256410 ║ 0,641025641025     ║
║        3 ║ James ║     450 ║ 0,230769230769 ║ 0,871794871794     ║
║        7 ║ Sarah ║     600 ║ 0,444444444444 ║ 0,444444444444     ║
║        5 ║ Sarah ║     500 ║ 0,370370370370 ║ 0,814814814814     ║
╚══════════╩═══════╩═════════╩════════════════╩════════════════════╝

SQL Server 2008 não suporta tudo em OVER() cláusula, mas ROW_NUMBER faz.

Primeiro cte basta calcular a posição dentro de um grupo:

╔═══════════╦════════╦══════════╦════╗
║ Customer  ║ User   ║ Revenue  ║ rn ║
╠═══════════╬════════╬══════════╬════╣
║        2  ║ James  ║     750  ║  1 ║
║        1  ║ James  ║     500  ║  2 ║
║        3  ║ James  ║     450  ║  3 ║
║        8  ║ James  ║     150  ║  4 ║
║        9  ║ James  ║     100  ║  5 ║
║        7  ║ Sarah  ║     600  ║  1 ║
║        5  ║ Sarah  ║     500  ║  2 ║
║        6  ║ Sarah  ║     150  ║  3 ║
║        4  ║ Sarah  ║     100  ║  4 ║
╚═══════════╩════════╩══════════╩════╝

Segundo cte:

c2 subconsulta calcula o total em execução com base na classificação de ROW_NUMBER
c3 calcular a soma total por usuário

Na consulta final s a subconsulta encontra a running mais baixa total superior a 80%.

EDIÇÃO 3:

Usando ROW_NUMBER é realmente redundante.

WITH cte AS
(
    SELECT c.Customer, c.[User], c.[Revenue]
           ,percentile         = 1.0 * Revenue / NULLIF(c3.s,0)
           ,running_percentile = 1.0 * c2.s    / NULLIF(c3.s,0)
    FROM t c
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM t c2
          WHERE c.[User] = c2.[User]
            AND c2.Revenue >= c.Revenue) c2
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM t c2
          WHERE c.[User] = c2.[User]) AS c3
) 
SELECT a.*
FROM cte a
CROSS APPLY (SELECT MIN(running_percentile) AS rp
             FROM cte c2
             WHERE running_percentile >= 0.8
               AND c2.[User] = a.[User]) AS s
WHERE a.running_percentile <= s.rp
ORDER BY [User], Revenue DESC;

LiveDemo4