Personally, I think 2 is actually your best option. The cons that you outlined I don't see as drawbacks, actually. You can easily add powerful join indexing simply by adding a mandatory index on each extended table on its foreign key:

--Main Table Sample:

CREATE TABLE MainTable

(

  accountID INT NOT NULL

, recordID INT NOT NULL

, companyname VARCHAR(100) NOT NULL

, CONSTRAINT PK_MainTable PRIMARY KEY (accountID, recordID) 

)

--Example Extended Tables

CREATE TABLE ExTable_Cust1

(

  accountID INT NOT NULL

, recordID INT NOT NULL

, someIntField INT NULL

, someTextField TEXT NULL

, CONSTRAINT FK_MainTable_ExTable_Cust1 FOREIGN KEY (accountID, recordID) REFERENCES MainTable (accountID, recordID)

)

--Index on Foreign Key

CREATE NONCLUSTERED INDEX IDX_ExTable_Cust1_MainTable ON ExTable_Cust1 (accountID, recordID)

This will give you a great indexing scheme for table joins, and your ease of maintenance will probably depend on how modular you make your logic to create the extended tables (ie with a standard suffix, etc...)

HTH,

Jay