CREATE TABLE #tmpChecks
(
OrgCheck char(15) PRIMARY KEY CLUSTERED,
OrgId smallint,
CheckNum int,
Status char(1),
AmtCheck money,
CheckDate char(10),
RegisterId char(12)
)

INSERT INTO #tmpChecks
SELECT CAST(OrgId as char(3))+PrintedCheckNum as OrgCheck,
OrgId, PrintedCheckNum as CheckNum, Status,
AmtCheck = CASE WHEN Status = 'V' THEN 0 ELSE AmtCheck END,
CONVERT(char(10), DateCheck,101) as CheckDate, RegisterId
from MyTable WITH (NOLOCK)
WHERE
ISNUMERIC(PrintedCheckNum) = 1
order by OrgCheck

IF OBJECT_ID('tempdb..#temp') IS NOT NULL DROP TABLE #temp;
CREATE TABLE #temp (acct_nbr INT, 
                    ckid INT, 
                    stat varchar(10), 
                    Grp int);
INSERT INTO #temp (acct_nbr, ckid, stat)
SELECT 1, 101 ,'Open' UNION ALL
SELECT 1, 102 ,'Open' UNION ALL
SELECT 1, 103 ,'Open' UNION ALL
SELECT 1, 104 ,'Void' UNION ALL
SELECT 1, 105 ,'Void' UNION ALL
SELECT 1, 106 ,'Open' UNION ALL
SELECT 1, 107 ,'Open' UNION ALL
SELECT 1, 108 ,'Open' UNION ALL
SELECT 1, 109 ,'Void' UNION ALL
SELECT 1, 110 ,'Open' UNION ALL
SELECT 2, 101 ,'Open' UNION ALL
SELECT 2, 102 ,'Open' UNION ALL
SELECT 2, 103 ,'Open' UNION ALL
SELECT 2, 104 ,'Void' UNION ALL
SELECT 3, 105 ,'Void' UNION ALL
SELECT 3, 106 ,'Open' UNION ALL
SELECT 3, 107 ,'Open' UNION ALL
SELECT 4, 108 ,'Open' UNION ALL
SELECT 4, 109 ,'Void' UNION ALL
SELECT 4, 110 ,'Open';

-- expected output:
SELECT acct_nbr = 1, ckid = 101 , stat = 'Open', grp = 1 UNION ALL
SELECT 1, 102 ,'Open',1 UNION ALL
SELECT 1, 103 ,'Open',1 UNION ALL
SELECT 1, 104 ,'Void',2 UNION ALL
SELECT 1, 105 ,'Void',2 UNION ALL
SELECT 1, 106 ,'Open',3 UNION ALL
SELECT 1, 107 ,'Open',3 UNION ALL
SELECT 1, 108 ,'Open',3 UNION ALL
SELECT 1, 109 ,'Void',4 UNION ALL
SELECT 1, 110 ,'Open',5 UNION ALL
SELECT 2, 101 ,'Open',1 UNION ALL
SELECT 2, 102 ,'Open',1 UNION ALL
SELECT 2, 103 ,'Open',1 UNION ALL
SELECT 2, 104 ,'Void',2 UNION ALL
SELECT 3, 105 ,'Void',1 UNION ALL
SELECT 3, 106 ,'Open',2 UNION ALL
SELECT 3, 107 ,'Open',2 UNION ALL
SELECT 4, 108 ,'Open',1 UNION ALL
SELECT 4, 109 ,'Void',2 UNION ALL
SELECT 4, 110 ,'Open',3;

IF OBJECT_ID('tempdb..#tempstore') IS NOT NULL DROP TABLE #tempstore;
CREATE TABLE #tempstore (acct_nbr INT, 
                    ckid INT, 
                    stat varchar(10), 
                    Grp int);
INSERT INTO #tempstore (acct_nbr, ckid, stat)
SELECT 1, 101 ,'Open' UNION ALL
SELECT 1, 102 ,'Open' UNION ALL
SELECT 1, 103 ,'Open' UNION ALL
SELECT 1, 104 ,'Void' UNION ALL
SELECT 1, 105 ,'Void' UNION ALL
SELECT 1, 106 ,'Open' UNION ALL
SELECT 1, 107 ,'Open' UNION ALL
SELECT 1, 108 ,'Open' UNION ALL
SELECT 1, 109 ,'Void' UNION ALL
SELECT 1, 110 ,'Open' UNION ALL
SELECT 2, 101 ,'Open' UNION ALL
SELECT 2, 102 ,'Open' UNION ALL
SELECT 2, 103 ,'Open' UNION ALL
SELECT 2, 104 ,'Void' UNION ALL
SELECT 3, 105 ,'Void' UNION ALL
SELECT 3, 106 ,'Open' UNION ALL
SELECT 3, 107 ,'Open' UNION ALL
SELECT 4, 108 ,'Open' UNION ALL
SELECT 4, 109 ,'Void' UNION ALL
SELECT 4, 110 ,'Open';

-- 400,000 rows of data
IF OBJECT_ID('tempdb..#temp') IS NOT NULL DROP TABLE #temp;
CREATE TABLE #temp (acct_nbr INT, 
                    ckid INT, 
                    stat varchar(10), 
                    Grp int);
INSERT INTO #temp (acct_nbr, ckid, stat)
SELECT acct_nbr+(4*n), ckid+(20*n), stat 
FROM #tempstore 
CROSS JOIN (SELECT TOP 20000 [n] = ROW_NUMBER() OVER(ORDER BY a.[name])-1 FROM master.dbo.syscolumns a, master.dbo.syscolumns b) n

-- copy our data to a working table  
SELECT rn = ROW_NUMBER() OVER (ORDER BY acct_nbr, ckid), 
	acct_nbr, ckid, stat 
INTO #Workings
FROM #temp
ORDER BY acct_nbr, ckid
CREATE UNIQUE CLUSTERED INDEX CIrn ON #Workings ([rn] ASC)

-- get the results
;WITH Calculator AS (
	SELECT rn, acct_nbr, ckid, stat, NewGroup = CAST(1 AS INT) 
	FROM #Workings WHERE rn = 1 
	UNION ALL 
	SELECT w.rn, w.acct_nbr, w.ckid, w.stat, 
		NewGroup = CASE 
		WHEN w.acct_nbr <> c.acct_nbr THEN 1 
		WHEN w.stat <> c.stat THEN c.NewGroup + 1
		ELSE c.NewGroup END
	FROM #Workings w 
	INNER JOIN Calculator c ON c.rn+1 = w.rn)
SELECT acct_nbr, ckid, stat, NewGroup 
FROM Calculator
OPTION( MAXRECURSION 0)
-- 400,000 rows: 15 seconds